Old hat reminder needed - What extension/compilation format to the mame roms need to be for retroarch? Much appreciated 
they need to be in a zip archive containing all of the bin/rom files.
There are several different versions of MAME. They are identified by their version, for example MAME 2003 uses v.78 ROMs .
since I am an idiot, I couldn’t figure out which core to download so I just downloaded them all. BUT I still cant get any of my mame rom’s to work at all.
Your ROMs need to match core version you’re trying to use: https://docs.libretro.com/guides/arcade-getting-started/
Thanks for the help - I forgot that .rom files had to be in the same zip folder
some arcade games have bios files of their own. I know Galaga, Donkey Kong but there is also the Sega STV bios for some arcade games such as Cotton 2 and Cotton Boomerang. Sega STV bios isn’t mentioned much.