On OpenBSD you can check the man page for fw_update IIRC. That utility is responsible for downloading firmware that couldn't be included in the kernel tree due to legal stipulations.

I went down the road recently to try and build a binary firmware free desktop. I gave up. I did manage a no non-free firmware build, which runs OpenBSD spectacularly, but I couldn't get rid of all firmware blobs.

It seems like open hardware is very hard to come by in this respect. Even swapping out your bios for coreboot is not possible for many (most?) systems--never mind all the other umpteen specialty computers running in your computer. A computer isn't one computer these days, and you will have a hard time taking a principled stand on all your software being free.