Search for lineage os your phone model if you're lucky someone will have ported the project to your device, the more popular it was the more likely
There are reasonable instructions
Your phone will need an unlocked bootloader, and it will not be able to be locked again while running lineage. This will prevent the use of any secure software which includes most bank apps, login authenticators. Local security is entirely bypassed by the unlocked bootloader
The frustrating thing is all the information and materials to support Samsung devices is out there, but the custom ROM development community is so ridiculously toxic and unhelpful to newcomers that few people are willing and able to mainline new devices (and supporting a new device is complicated enough that you pretty much can't get started without someone more experienced to point out the decades of cruft, pitfalls, and vendor-specific workarounds that the build processes have accumulated). More of the old guard leaves every year without enough new blood to replace them.
I like what GrapheneOS is doing, but it's focused on having the absolute best security rather than supporting a lot of phones. If you aren't wanted by three letter agencies then LineageOS is good enough
I bought my current phone specifically because it was supported by LineageOS. Before that I used an unsupported Samsung phone, which was such a shitty experience that I made a promise to myself that my next phone has to have custom ROMs available
I don't even pay any attention. I boot the phone for the very first time, go through the setup thing, and click no to as many things as possible, get into the home screen, open settings, open developer settings, turn on USB debugging, and turn the phone off and install lineage OS with no Google apps.