FAT12, and no it doesn’t work natively. I know this because I had to replace a floppy to fix a 40 year old computer earlier this year.

You can get a USB 3.5" floppy drive working with just some special software, but a 5.25" FDD was a huge pain involving open source hardware (greaseweazle) that reads the raw magnetic flux values that then have to be run through another janky piece of software to interpret it.