GrapheneOS
https://en.wikipedia.org/wiki/GrapheneOS
https://grapheneos.org/
https://www.reddit.com/r/GrapheneOS/
https://github.com/GrapheneOS/
LineageOS
https://en.wikipedia.org/wiki/LineageOS
https://lineageos.org/
https://www.reddit.com/r/LineageOS/
https://github.com/LineageOS
Replicant
https://en.wikipedia.org/wiki/Replicant_(operating_system)
https://www.replicant.us/
https://www.reddit.com/r/ReplicantOS/
https://git.replicant.us/replicant
Ubuntu Touch
https://en.wikipedia.org/wiki/Ubuntu_Touch
https://ubuntu-touch.io/
https://www.reddit.com/r/Linux/ and https://www.reddit.com/r/Ubuntu/
https://github.com/ubports/ubuntu-touch
There's also a couple of great "Beginner's Guides" available here: https://www.androidauthority.com/lineageos-install-guide-893303/ and https://www.howtogeek.com/348545/how-to-install-lineageos-on-android/. They are written with Lineage OS in mind, but are largely applicable to any custom OS. They nicely talk you through the processes of installing the necessary software on your computer, downloading the necessary software for your phone, backing up your phone, and flashing your phone.