Why do we assume that PHB rules apply to monsters?

Question

The party is deep into my 5e-updated classic Greyhawk Giants series.

They are returning the body of a stone giant killed in the siege of Headwater to the stone giant's clan. The Thane of the Stone Giants intends to hold Funerary Games in honor of his slain kinsman and invite the PC's to participate. One of the games will be unarmed combat.

In calculating an unarmed strike from a stone giant, I understand that the damage from the strike will be 7 points (1+ the stone giant Str mod). For the attack roll, however, I was unsure of whether to give the stone giants their proficiency bonus.

This question about unarmed bugbears, this question about unarmed skeletons, and this question about unarmed Flameskulls (?!), all have answers which state that monsters get to add their proficiency bonus to unarmed attacks, with the last question's accepted answer stating as a general principle that "creatures always have proficiency with their unarmed strike".

As justification for this position, these answers cite some variation of the Player's Handbook, PHB errata, or D&D Beyond, quoting "You are proficient with your unarmed strikes." None of the comments question this justification, and some support it.

Perhaps it is because I came to 5e from earlier editions, but my working assumption is that monsters / NPC's have stat blocks, not character levels, and don't necessarily follow the rules in the PHB. I read "You are proficient" and I feel like 'you' means 'You PC's', not 'All creatures'. I don't see anything in the Monster Manual that says that monsters in general follow the rules of the PHB or are proficient in unarmed strikes; I do see sidebars noting that monsters are proficient with their "armor, weapons, and tools" and that grappling rules work differently for many monsters compared to PC's.

Under the general principles of "there are no secret rules", and "abilities do what they say they do", where can I find a general statement saying that monsters follow the rules of the PHB, and that such "you" statements apply to them as well?

Related: Are monsters subject to the massive-damage instant-death rules? The topmost answer to this question is "Combat rules apply to everyone". The second most upvoted answer is "Rules only apply to monsters if the DM wants them to." This doesn't seem like the consensus that greets the unarmed strike rule, which is why I wonder if this is indeed a general principle.

Related: Is the telepathy rule in the Monster Manual only applicable to monster telepathy abilities?

Answer 1

If monsters don't follow the "you" rules in the PHB, then many things become undefined.

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

Can monsters make opportunity attacks?

You can avoid provoking an opportunity attack by taking the Disengage action.

Can monsters disengage?

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple.

Can monsters grapple (when they don't have a special-case grapple attack in their stat block)?

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.

Can monsters be surprised?

On your turn, you can move a distance up to your speed and take one action.

What can monsters do on a turn?

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Thankfully, we have a clear answer in the intro to the combat chapter.

This section provides the rules you need for your characters and monsters to engage in combat, whether it is a brief skirmish or an extended conflict in a dungeon or on a field of battle. Throughout this section, the rules address you, the player or Dungeon Master. The Dungeon Master controls all the monsters and nonplayer characters involved in combat, and each other player controls an adventurer. “You” can also mean the character or monster that you control.

Answer 2

The Monster Manual explicitly says so

Monsters can perform actions from the PHB:

When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player's Handbook.

MM p. 10

Monsters' spellcasting works as described in the PHB:

A monster with the Spellcasting special trait has a spellcaster level and spell slots, which it uses to cast its spells of 1 st level and higher (as explained in the Player's Handbook).

MM. p.10

Monsters are affected by proficiencies the way the PHB says:

Assume that a creature is proficient with its armor, weapons, and tools.

See the Player's Handbook for rules on using armor or weapons without proficiency.

MM p. 9

Monsters' size and speed work the same way as described in the PHB:

The Size Categories table shows how much space a creature of a particular size controls in combat. See the Player's Handbook for more information on creature size and space

A monster's speed tells you how far it can move on its turn. For more information on speed, see the Player's Handbook

Answer 3

Part 2 of the Player's Handbook explicitly applies to monsters. Other matters often reference back to the Player's Handbook

Player's Handbook

'Chapter 7: Using Ability Scores' and 'Chapter 8: Adventuring' regularly specify their rules as applying to both characters and monsters, or use the term "creatures" which is inclusive of both characters and monsters. 'Chapter 9: Combat' typically uses the term "you", but specifies that

"You" can also mean the character or monster that you control.

Whether this can be generalised to other chapters or only applies to Chapter 9 is ambiguous from this sentence alone, though. Regardless, this does answer whether "You are proficient in unarmed strikes" (PHB p. 195 (Chapter 9)) applies to monsters.

Regarding 'Chapter 10: Spellcasting',

Regardless of its source, a spell follows the rules here.

As far as Part 2 of the Player's Handbook is concerned, it explicitly applies to monsters. Rules from Part 1 will rely on slightly less direct inferences.

Monster Manual

The introduction to the Monster Manual regularly repeats the phrase "See the Player's Handbook" when referring to some rule not defined in the Monster Manual. While lacking explicit page references, many of these cross-references point to Part 2 of the Player's Handbook. However, there are a few exceptions.

See the Player's Handbook for descriptions of the different alignments.

C.f. PHB p. 122

See the Player's Handbook for rules on using armor or weapons without proficiency.

C.f. PHB p. 144, 146

You can equip monsters with additional gear and trinkets however you like, using the equipment chapter of the Player's Handbook for inspiration

(Granted, "for inspiration" makes this a soft reference. Monsters can use equipment not found in the Player's Handbook.)

The Monster Manual also mentions ammunition for ranged and thrown weapons which need them, implying that monsters using weapons with the Ammunition and Thrown properties as described in the Player's Handbook follow those rules.

The rules for weapon and armour proficiencies, alignment, and equipment exist in the Player's Handbook and are explicitly referenced by the Monster Manual.

Dungeon Master's Guide

The 'Creating a Monster' section (page 273 onwards) contains other references to the Player's Handbook regarding equipment (as well as references already made in the Player's Handbook or Monster Manual).

When making a monster, there is guidance on what to do if the monster uses manufactured equipment.

If your monster wears manufactured armor, its Armor Class is based on the type of armour worn (see the Player's Handbook for armor types).

If a monster wields a manufactured weapon, it deals damage appropriate to the weapon.

The damage of most weapons is defined in the Player's Handbook.

Furthermore, under 'Switching Weapons' (p. 273),

Don't forget to change the damage and the attack's reach where appropriate. Also be aware of the consequences of switching from a one-handed weapon to a two-handed weapon, or vice versa.

These statements all imply that monsters, by default, follow the rules for equipment as defined in the Player's Handbook when they use equipment. Of course, this section of the Dungeon Master's Guide is all about customisation, and it provides options for determining damage and armour which ignore typical equipment rules. But where a monster does use equipment, it is typically assumed (although not guaranteed) that it follows the rules of that equipment.