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 by default from the manufacturer. 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 sandboxing through user profiles, and remains supported while phones are still under Google's "seven years of updates" coverage.
• e/OS is a degoogled fork of LineageOS
• CalyxOS is treated as a competitor to Graphene, with same support length and similar devices. [wp>/wiki/CalyxOS|Wikipedia link]]
• DivestOS is a discontinued soft-fork of LineageOS, built to support end-of-life devices.
• PostmarketOS is Alpine-based Linux operating system, built explicitly for end-of-life devices: phones, tablets, computers. See their device list.

Samsung Galaxy A21, which was successfully charged and booted for the first time since 2024. Planning to test some custom roms. Open to your suggestions!

• LineageOS
• crDroid

• PostmarketOS is unsupported
• GrapheneOS and CalyxOS are a no.

Let me know!