The difference between BusinessFirst and Domestic First and in both soft and hard product.

The soft product is vastly different (more, larger, and better meal options, more wine choices, more FAs, amenity kit, lounge access... basically everything one would expect from a premium international cabin, just like you would get with international F/C on UA, as opposed to domestic F on UA).

For hard product, sometimes, domestic F will be in BF seats, but usually not. When it's not, domestic F is a big leather seat, 38 inch seat pitch, more recline, wider seat...
BF on 752/772 is completely lie-flat, akin to the new C on UA, but wider.
The 764 is a recliner seat, but a lot wider, a lot more legroom, and a lot more recline. (And is getting the new lie-flat seats anyways)

Why do you say it's not lie-flat? Most is