I can suggest the xda-developers site. All roms out there are open source. For example, in this link you can download the lineage 18.1, which is based on Android 11:
https://xdaforums.com/t/official-lineageos-18-1-for-the-samsung-galaxy-s4-all-qcom-variants.4255313/
I first need to root the device? I tried installing Lineage but signature verification failed. I assume because it’s not rooted.
yes. in that site should be inttruction to install custom rom, You must read the instructions carefully and follow them step by step. Most of the time, installing custom rom consists of these steps: 1- Rooting the phone 2- Installing custom recovery 3- Installing custom rom through custom recovery.
