###Monsters
What counts as statistics for monsters (at least) is defined in the Monster Manual introduction in the section "Statistics".
It begins by saying:
A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster. (MM 6)
and then proceeds to dedicate a section to describing each aspect of the statistics in a short sub-section (MM 6-11).
- Size
- Type
- Tags
- Alignment
- Armor Class
- Hit Points
- Speed
- Ability Scores
- Saving Throws
- Skills
- Vulnerabilities, Resistances, and Immunities
- Senses
- Languages
- Challenge
- Special Traits
- Actions
- Reactions
- Limited Usage
- Equipment
- Legendary Monsters: Legendary Actions
Monsters, by default, are not given class levels and thus no section was dedicated to talking about them as part of the statistics. However, a later section talks about the option of adding class levels to monsters:
You can use the rules in chapter 3 of the Player’s Handbook to give class levels to a monster. For example, you can turn an ordinary werewolf into a werewolf with four levels of the barbarian class (such a monster would be expressed as “Werewolf, 4th-level barbarian”).
Start with the monster’s stat block. The monster gains all the class features for every class level you add, with the following exceptions:
The monster doesn’t gain the starting equipment of the added class.
For each class level you add, the monster gains one Hit Die of its normal type (based on its size), ignoring the class’s Hit Die progression.
The monster’s proficiency bonus is based on its challenge rating, not its class levels.
Once you finish adding class levels to a monster, feel free to tweak its ability scores as you see fit (for example, raising the monster’s Intelligence score so that the monster is a more effective wizard), and make whatever other adjustments are needed. You’ll need to recalculate its challenge rating as though you had designed the monster from scratch.
Depending on the monster and the number of class levels you add to it, its challenge rating might change very little or increase dramatically.
###NPCs
An NPCs stats can be generated in several different ways:
When you give an NPC game statistics, you have three main options: giving the NPC only the few statistics it needs, give the NPC a monster stat block, or give the NPC a class and levels. The latter two options require a bit of explanation.
Using a Monster Stat Block
The Monster Manual contains statistics for many generic NPCs that you can customize as you see fit, and chapter 9 of these rules offers guidelines on adjusting their statistics and creating a new stat block.
Using Classes and Levels
You can create an NPC just as you would a player character, using the rules in the Player’s Handbook. You can even use a character sheet to keep track of the NPC’s vital information.
Class Options. In addition to the class options in the Player’s Handbook, two additional class options are available for evil player characters and NPCs: the Death domain for clerics and the oathbreaker for paladins. Both options are detailed at the end of this chapter.
Equipment. Most NPCs don’t need an exhaustive list of equipment. An enemy meant to be faced in combat requires weapons and armor, plus any treasure the NPC carries (including magic items that might be used against the adventurers).
Challenge Rating. An NPC built for combat needs a challenge rating. Use the rules in chapter 9 to determine the NPC’s challenge rating, just as you would for a monster you designed.
Regardless it is clear that what counts as statistics for an NPC are the same for what counts as monsters and PCs. However, the DM, for simplicity's sake, just has a lot of leeway on how many of those statistics they need to define when making an NPC.