ok, I see how reckless attack increases damage output at the higher ACs. I've never understood how Crossbow expert allowed for high damage output because I always felt you needed a free hand to load the crossbow so I never really considered it. But that's a subject for another thread (or has been the subject of countless threads)
XBE says "if you attack with a weapon, you can make an attack with a hand crossbow as a bonus action".
The weapon that triggers the bonus action attack... can also be a hand crossbow. The SAME hand crossbow.
So you use a hand crossbow, and a hand free. The free hand reloads the hand crossbow. Loading a weapon is part of its attack. You may need to use your free object interaction to have it loaded before you do your bonus action attack.
---
Magic-item less martial DPR in 5e is about taps times damage per tap.
SS or GWM boosts your damage per tap.
XBE or PAM boosts your taps.
The +10 from GWM/SS overwealms die size.
Now, a polearm (1d10 main, 1d4 bonus) is better than a hand xbow (1d6 main, 1d6 bonus), but only marginally.
Melee weapon's main edge on ranged weapons is the existence of girdles of giant strength, which permit strength over 20, and flametongue weapons. There is no equivalent for dex builds.
Ranged weapons get magic arrows.
A flametongue polearm is 12.5 main/9.5 bonus at +0 to hit. Throw on 29 strength girdle (+9/+9) and GWM (-5/+10) and you are at +10/(31.5/28.5) ATK/damage, plus a few points from GWF style.
A +3 hand crossbow with +3 bolts is 9.5 main/9.5 bonus at +6 to hit. 20 dex (+5/+5) and SS (-5/+10) and archery (+2) style you are at +14/24.5.
A fighter's attack round is then +10 to hit for a total of 154.5 damage on polearm, or +14 to hit for a total of 122.5 on hand crossbow.
Note that flametongue compared to a +3 weapon is a -3 to hit for +4 to damage. It acts like a "mini -5/+10" trade of accuracy for damage, but is less efficient. It is also as rare as a +2 weapon, so it is more like -2 to hit for +5 to damage, which is a better ratio than -5/+10. It also deals extra crit damage.
So the XBow has very rare hand xbow and many, many consumable very rare +3 arrows. The melee build has a legendary girdle and a rare polearm.
There isn't a +damage weapon on the scale of a +3 weapon (very rare); in theory it could deal +3d6 damage (10.5) for -3 to hit for +7.5 to damage.
---
The Barb9/Fighter11 version drops from 5 to 4 swings in exchange for increased accuracy and crit chance on those swings (plus some crit damage and flat damage from rage).
This makes the penalty to-hit from GWM less important, and lands more taps. But a girdle of giant strength also does this almost as well.