Free replacements for the Android operating system are come by if you have a specific device in mind. You have to root your device, install a new ROM, etc. And many devices require many nonfree blobs for your system to operate.
If you want a mostly free operating system for Android, LineageOS is your best bet. Replicant is the fully free fork, but it is very unlikely to work on your device.
Good luck rooting! Please say how it turns out.
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