Do the Hunter ranger's Extra Attack, Whirlwind Attack, and/or Horde Breaker features work together?

Question

At level 5, all rangers get the Extra Attack feature:

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

At level 3, the Hunter ranger archetype gets Hunter's Prey, which lets them choose from 3 possible features. One option is Horde Breaker:

Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

At level 11, Hunters gain the Multiattack feature, which lets them choose between the Whirlwind Attack and Volley options. Whirlwind Attack says:

You can use your action to make a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.

Am I right that Whirlwind Attack can not be used with Extra Attack, because you do not use the Attack action to make a Whirlwind Attack? Generally, can Extra Attack be used with any of the Hunter ranger's special attacks?

I think with Horde Breaker, the "different creature" clause excludes any creature originally targeted with Whirlwind Attack. This means unless you have a reach weapon, Horde Breaker is useless with Whirlwind Attack. Am I mistaken?

Answer 1

Extra Attack only modifies a regular Attack action

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

I think the fact that "Attack" is capitalized seems to implicate the Attack action under actions in combat vs. any attack.

Horde Breaker could be used in conjunction with Whirlwind Attack

Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

Since Horde Breaker triggers off a "weapon attack" not an "Attack action" you should be able to use Horde Breaker to target one of the enemies next to you. Its simply triggering off one of the attacks made as part of the whirlwind.

Optimization breakdown

Since all Hunters get either Volley or Whirlwind, the Extra Attack class feature is there as a stopgap in the meantime and also a fallback for when there is only one enemy in range to be targeted. Horde Breaker should still be viable even though its utility will diminish in comparison upon reaching level 11 as a ranger, but even then its still one more attack per turn.

Answer 2

Correct you can't use extra attack and whirlwind attack on the same turn. You can see this more clearly in the definition of "Attack Action"

The most common action to take in combat is the Attack action, wheter you are swinging a sword, firing an arrow from a bow, or brawling with your fists. With this action, you make one melee or ranged attack.

"melee attacks" are different from "Attack action", and whirlwind attack is instead of Attack action.

For Horde Breaker, you can use it with whirlwind attack, you just can't hit the same creature twice when activating hoardbreaker. The fact that they were targeted with a previous whirlwind attack is irrelevant. Say you have 3 Goblins in range. You decide to use, Whirlwind attack to attack all 3. Your first hit against goblin 1, allows you to now use hoardbreaker against goblin 2. You then hit goblin 2 and goblin 3 with whirlwind attack. (Since hoardbreaker can only be used once per turn)