

You have to be careful to get a phone and model supported by one of the projects. Check all compatibility and install instructions before buying a phone. And if you need a manufacturer supplied unlock code, make sure the manufacturer still gives them out . Some will discontinue that service after a few years.
For graphene os you need one of the gogle devices - i’ve never tried it but i think its the one most people like.
lineageos supports more devices usually older.
I recently got lineageos working on sony experia xa2 - very happy with it. But to get there i had to go try like 6 computers before one of them sucessfully sent the bootloader unlock code over the ADB. For some reason usb is temperamental when doing stuff like that
It is a lot easier on really old stuff like samsung galaxy s3 or s4 if you can tolerate something that old. Maybe you’ll lso end upon an old version of lineage.
Once you get the bootloader unlocked it is generally straightforward. but modern phones make that fist part awkward.
Adverts for public transport? Makes about as much sense as adverts for fossil fuel - I guess. I don’t recall ever seing one where I live though.