I think btrfs was the default the last time I installed Bazzite, but I don’t really know anything about it so I switched it to ext4. I understand the snapshot ability is nice with rolling release distros, though.
It’d been ages since I’d used FAT32 for anything until I made a Debian live USB when I was setting up my pi-hole on an old Core2Duo recently. It would only boot on FAT32 for reasons I probably once knew. 😆
NTFS was an improvement over the FATs what with the journaling, security, file streams, etc. I use it wherever I still use Windows (work).
Most of my general purpose USB flash drives use exFAT. I like not having to worry about eject/unmount.