New to this and struggling a little bit. I have a FSA MegaEvo bb-8000/6000 installed on my bike. I am trying to figure out what cranksets are compatible with this BB.

I think I found the exact one on tradeinn.com but I can’t link it since I’m new.

Currently there are Shimano FC R-770 installed but I got this used so not 100% sure if those are actually compatible or not. The listing I found on tradeinn says I can install BB386EVO crankset. Is this just a proprietary crank interface? It also says 68mm BB shell and the Shimano technical document on the R770 crank says it has a 68mm shell width. Does that mean I can install any crank with 68mm shell width?

I know it would probably be easier to replace the BB since it is BSA but I’m also just trying to learn about crankset compatibility somewhat.

Do you mean an FC-R700?

Think there’s a good chance any shimano road 24mm diameter spindle (which they all are) hollowtech ii crank will fit.

Trying to find a crankset to match an existing bottom bracket is going about things backwards

I've found FSA BBs to be considerably less durable than Shimano's, and considerably more expensive. They are also a couple millimeters narrower. Personally, I'd just keep using the Shimano crank and replace the BB with whatever Shimano's offering now for Hollowtech II for 68 mm and BSA. If you want to use this BB until it fails, I'd suggest a couple a couple mm of spacer behind the drive side bearing to correct the chain line.

As I understand FSA, Mega EXO cranks/BB have a 24mm diameter spindle where Mega EVO crank/BB has a 30mm spindle.
Shimano is strictly 24mm spindle so I wonder about the OP EVO...... whether it is really an EXO or maybe there are shims involved?

The BB6000 is a MegaEXO according to all that is Google

BBs are cheap, cranks are not. Your issue is simple and the general rule for this: get the crank you want and a BB to match it and the frame's BB shell (i.e.threaded, press fit, etc), not the other way around.