[RPG] How does the Ranger’s Extra Attack feature work with Two-Weapon Fighting

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.

You’re missing the two-weapon fighting combat option rules. Without those, the feat et al. won’t make very much sense.

The two-weapon fighting combat option allows you to get an extra attack when you full attack, but all of your attacks in that full attack take a penalty. The extra attack has to be with a different weapon from your regular attack (“offhand” weapon), and the penalties you take depend on whether or not that offhand weapon is light or not.

The Two-Weapon Fighting feat reduces those penalties to a point that this option is actually usable.

Another key thing to realize is that having two weapons in hand does not automatically mean you are using the two-weapon fighting option. You have to choose to use it, to get an extra attack and take an attack penalty.

As for the +6/+1 thing, that’s called an iterative attack, and that’s free just for having BAB that high. It is separate from, and can be combined with, the two-weapon fighting option. So, assuming you have the Two-Weapon Fighting feat and a light weapon in the offhand (the only case in which the two-weapon fighting option is a good idea), with +6/+1 BAB you could attack normally, for +6/+1, or you could choose to use two-weapon fighting, taking a −2 penalty. Then your attacks would be +4/+4/−1, where the second +4 attack has to be with the offhand weapon. If you take the Improved Two-Weapon Fighting feat, that becomes +4/+4/−1/−1, because Improved Two-Weapon Fighting gives you a second bonus attack with the offhand weapon, at −5 (matching the penalty on the iterative attack).