It doesn’t matter that it’s on different drives. Whichever OS is running has full hardware access.
Windows is notoriously bad at not touching other people’s stuff. It’s not unheard of for it to erase Linux’s bootloader.
If that’s all it did, arch-chroot https://discovery.endeavouros.com/system-rescue/arch-chroot/2022/12/ followed by GRUB reinstall (or systemd-boot? EOS switched to the latter a year back or so; what you have depends on when you first installed EOS) might suffice to fix it.
Don’t hesitate to make a new post about this problem, since it’s become much more serious than Steam games not booting, and the current thread title does not convey that.
Good luck.