The most known Operating Systems for mobile devices ("smart"phones, tablets) and TVs are Android, iOS and Linux.
The latter can run on a number of devices, but gives the best results on devices with no proprietary firmware and open hardware, like the PinePhone and similar products.
iOS is quite hopeless and the hardware is locked to that Operating System. There's not much that can be done (with any Apple product).
Android as sold commercially is almost always a version with embedded Google services, that can't be used without a Google account (apart from some Chinese devices, that have though other kind of issues).
Android in its Open Source version (AOSP) has some limitations due to the closed-source firmware running under the hood (on modems, wifi/bluetooth/gps cards etc) but is still much better than any "normal" phone (Android with Google or iOS).
The two most viable alternatives, available for a good number of devices, are:
Get in touch for more info, I am using Lineage and Linux phones since 2018 and will never go back.