Android is built on the AOSP project. You can access your phone, or tablet, using the Android SDK to access Android Debug Tooling, adb, tooling in the terminal. This is how you would commonly perform basic backups, access device folders in your file manager, and perform rom flashing.

Stock being the system how it arrives. Alternatives include:

• LineageOS, which most other projects are based off.
• crDroid, a feature rich variation of LineageOS
• GrapheneOS, designed almost exclusively for the Google Pixel line. Fork of Copperhead, built for security and excellence while phones are still supported under Google's seven year coding coverage.