This is a Database that will help you get a few console and computer BIOS that are compatible with MiSTer cores.
If you use Update All, you may simply activate it there in the Settings Screen.
Alternatively, if you don't use it, you can add an entry to the downloader.ini file on the root of your SD (create it if it doesn't exist), to use it with the MiSTer Downloader (which is what Update All calls under the hood). For that, please add the following entry there:
[bios_db]
db_url = https://raw.githubusercontent.com/theypsilon/BiosDB_MiSTer/db/bios_db.json
Once you've done this, next time you run downloader, the BIOS will get installed too.