For those new to the process, the best general resource for instructions on how to unlock your phone’s bootloader and flash alternate AOSP ROM’s or non-Android OS’s is https://xdaforums.com/ Please note that not all carriers and oem’s allow you to unlock the bootloader though, so choose your device carefully for this.

You will also most likely need a PC (desktop or laptop) with adb & fastboot on it. These are apps used in the terminal, but you only need to copy and paste a few commands into them to use it.

If you have a Google Pixel then best option is Graphene - https://grapheneos.org/

For other devices you can use a “degoogled” Android ROM and get apps from the open source F-Droid app store - https://f-droid.org/ Some choices for this are: Lineage - https://lineageos.org/ crDroid - https://crdroid.net/ /e/os - https://e.foundation/e-os/ Iode - https://iode.tech/iodeos/

OR use a a true alternative mobile OS. Options for this are: Ubuntu Touch - https://www.ubuntu-touch.io/ Sailfish OS - https://sailfishos.org/ Mobian - https://mobian-project.org/ Postmarket OS - https://postmarketos.org/ Plasma Mobile - https://plasma-mobile.org/ Droidian - https://droidian.org/

You can also purchase devices with alternative OS’s already preinstalled from: Volla - https://volla.online/en/devices/ Jolla - https://jolla.com/ Fairphone - https://fairphone.com/ Murena - https://murena.com/ Furilabs - https://furilabs.com/ Brax - https://www.braxtech.net/