Just FYI, since nobody responded, the steps to check:

1. Search if the phone model can have the bootloader unlocked. If no, then you’re likely unable to do anything. Note that in some cases, this can vary by region. (Ex. the US version could have a locked bootloader, but the international model can be unlocked). The bootloader is basically the software that loads your phone operating system, and it needs to be unlocked to allow the OS to be changed.
2. LineageOS is probably the easiest to check for a reputable replacement OS, for non-Pixel phones (for which you can get GrapheneOS.)
3. XDA Forums used to be the #1 spot for phone “ROMs” (alternative OSes); it’s changed a bit, afaik, but I think they’re likely the next easiest spot to look for a custom OS, if GrapheneOS and LineageOS isn’t an option.

A few Linux phones work, but there are only a handful of models supported. If you don’t have a Fairphone 5 or a phone specifically marketed/sold as a Linux phone, you’re most likely out of luck, unless you’re willing to make major compromises, like non-working cell connectivity, camera, etc.