What operating system to install on it

Computer:

• A very simple, easy to use and common linux distribution is ubuntu.
• If your computer is old or slow, you can try lubuntu or xubuntu which are very light.
• ElementaryOS, linux with simple and sleek interface.

Phone

• LineageOS, a libre version of android.
• /e/, an easy and simple version of android (you can also buy from them second hand phones with it already installed).
• Copperhead OS, a security and privacy focused mobile operating system compatible with Android apps.
• GrapheneOS, an open source privacy and security focused mobile OS with Android app compatibility.
• Replicant, a fully free Android distribution, this means that if some part of your phone don't have free drivers, they will just not work.
• Phone systems not based on android: Plasma Mobile, PostmarketOS, Ubuntu Touch...