If you choose a operating system software that is not owned by a big tec company – you are already steps ahead. For smartphones we recommend
E/OS or
Graphene OS (the camera app on the Fairphone is way better on e/0s).For desktop we recommend
Fedora (Linux based) or
Cube OS. If you want to have a laptop on a usb stick see
Tails.It is easy to buy a device that has the software already – which is possible with Fairphone and Framework. But doing it yourself is not so hard – especially with the e/os
tutorial where you can install it via your browser.For Apple devices there are no different operating systems available.