Do you have an old, unused Android phone lying around?  

By following instructions at the links below you can "root" your phone, and put on a customized version of Android.  

Have fun and learn a lot at the same time.

RootJunky - Huge source of YouTube videos on all kinds of Android phones, with step-by steps showing how to root, install custom ROM's, etc.

LineageOS Android Distribution - the most popular non-Google version of Android out there.  A free and open-source operating system for various devices, based on the Android mobile platform

XDA - the go-to source for all things Android. Create an account, make contributions, and become a senior member.

Android Forums - another source of technical information through forums.

YouTube Video: Android phones 101 / android for dummies.