@Maker_of_Things
Unfortunately, those are two different problems that are often contradictory.
Installing a different OS is gated by a few factors: does the alternative OS support your phone (often not), is your phone not locked to your carrier (often is), and are you able to unlock the boot loader to allow the install (again, often not).
You then have the requirement of having another computer that can make a USB connection to the phone; installs (in my experience) cannot be performed over the air. It often requires using an application from the OS supplier, one that runs from their website, or installing the Android Debug Bridge (ADB), which is free, but is operated as a command line application. There are always instructions and tutorials on these, but I've often found it's spread across different sites.
The second part is highly dependent on whether you are trying to leave the Google environment or not.
For example, there is an app called SMS Backup & Restore, which can, as it's name implies, back up all your text messages to a file and restore them later. But that file will have to be saved somewhere (online, or on another computer), because nothing survives the installation process.
If you're staying with Google, then Gmail and Google Photos can be backed up and restored using your Gmail login. But if not, it can be problematic to move email to a different application. Photos can be transferred to a computer over USB, then copied back after the install if you don't want to use Google Photos.
Other apps again may depend on the presence of the Google Play Store on your phone, requiring a Google sign-in.
Many alternative OS's provide F-droid, an alternative app store, but many commercial apps aren't on it.
Again, all apps won't survive the install, so would need to be reinstalled, unless backed up and restored from the Googleverse.
