best format for thumb drive

You can't share a photos library with windows. there is no app to open it. And there is no windows compatible format that is suitable for a Photos library.

If you are talking about a thumbdrive just to put individual image files on, then yes, ExFat will be fine.

Bear in mind you can't put a photos library on a thumbdrive, the best format would be MacOS extended or APFS - assuming you don't need to share it with a windows machine.

> share with windows?

exFAT.

What if I do want to share with windows?