A number of phones have official support for LineageOS (popular FOSS custom OS very close to stock FOSS Android), in which case installing the OS is a simple and straightforward process with no extra binaries required except those published by the LineageOS project. It's very common to also use the TWRP recovery but actually the process is a bit easier with the official LOS recovery.
After that root is provided with Magisk, which is also open source. And you could also install the Google software package if you want at this stage to get Play Store / etc.
Plus, with LOS you get guaranteed security & version updates for Android for years and years and years. Unfortunately firmware blobs for the hardware are provided by the OEM, and while LOS tries to keep them updated sometimes they are a bit old, and updates for those will eventually end when the OEM stops providing updates.
After that root is provided with Magisk, which is also open source. And you could also install the Google software package if you want at this stage to get Play Store / etc.
Plus, with LOS you get guaranteed security & version updates for Android for years and years and years. Unfortunately firmware blobs for the hardware are provided by the OEM, and while LOS tries to keep them updated sometimes they are a bit old, and updates for those will eventually end when the OEM stops providing updates.