Bf4 reply - maybe, I dunno.
Why is this the 'best move'?

Yeah,
Nxe5...Nxe5.
Bf4...
Both pawns pinned - looks like some nasty fireworks are possible. Look at some line and then run it through self analysis.

This is very tasty. I think it might be looking at something like this if N was to retreat
Nxe5...Nxe5.
Bf4...Nf6
Nc7+...Ke7
Qd6#

When your opponent is underdeveloped and king is in the centre and most of your pieces are ready for action , that is the time for sacrificing pieces to open up files and diagonals and ruin the king safety as much as you can. But, it can be risky if you don't know the proper follow up and end up losing sufficient resources for the fireworks. I can see that black is really underdeveloped and whites Knight on d5 is controlling a lot of squares close to blacks king. Especially the dark squares. There are possibilities of forks and checkmates. So if white can somehow bring his other pieces quickly before black comes up with a defense, white can deliver a good attak. I guess sacrificing the knight is the quickest way too bring the dark square bishop in to the attack without losing tempo. Even if black doesn’t take the knight and instead takes the light square bishop, the knight can also play part in attacking combinations. But, the question is whether the combination works or not. That is something we have to calculate and check all the candidate moves.. So, I guess for a beginner, it’s okay to play safe and defend the bishop. Computer is considering it a mistake because it calculated all the moves and confirmed that there are other moves that has more winning chances and considering defending the bishop as a missed opportunity. For humans or average humans, we don't have to take risks because we can't calculate all the way to the winning position and confirm. Especially if it’s a blitz game. So It's okay to play safe even if it means you missed a long combination that leads to a better position.

yeah Bf4 if you try to defend the knight with something like ...f6 you get BxN and then Qh5 is coming. Black is in trouble.

This position is rich with tactics . I like it.
I am just testing how good Leela Chess Zero in my phone , whether she see tactical lines, I am surprised she can see those tactics like Stockfish.

Honestly, I'm newish to chess, still pretty don't know terms like "Stockfish", I have some reading to do. As for my position with LSB on b5, we were fighting a bit over the d column and I was messing with the idea of harassing the queen and pawn in that area with Nf6. It probably would have ended in a bad trade, but like I said, I'm not that good at chess yet.
Why wouldn't I back my bishop away? This sacrifices my knight (or my bishop, it's opponent's pick essentially) and gives my opponent more control of the center if they opt for my Knight, and then it still doesn't address my exposed Bishop.. When I protect my Bishop, it cries blunder.
I'm still new, so I may be missing something key here.