What operating system should I use?
For privacy, you should always avoid using Windows as an operating system.
With that being said here is the list of operating systems I recommend:
For computers, I would use Fedora or Linux Mint. Use TailsOS if you need something more intense that won't remember what you do and runs all internet traffic strictly through the Tor network.
For mobile devices, I would use GrapheneOS, CalyxOS, or LineageOS. It all depends on your needs and threat model, so research as needed.