For example: https://phtn.app/post/lemmy.ml/26299109

There are a couple of other posts, you’ll find them quickly if you search for Pewdiepie.

LineageOS has instructions for Windows/Mac/Linux: https://wiki.lineageos.org/adb_fastboot_guide

I personally wouldn’t downgrade to Android 11 since the Lineage Installation Instructions clearly state you need to be on 12.

If there is an error during unlocking the bootloader, I would get in touch with the people from LineageOS - scroll to the end of the same page “Get assistance”.

In Mint most people recommend installing Steam directly from the website as .deb. There are dozens of informative threads on the official forums, which also explain some differences of the installation method, for example: https://forums.linuxmint.com/viewtopic.php?t=427582