@roger-ramjet Have a look here and read through the link in the first post https://forums.ubports.com/topic/7568/install-ut-on-google-pixel-3a-xl/29 See if it makes things clearer.
You will need a specific version for your device but the instructions should make that clear and have all the links you need. Make sure you take your time and you will be ok. Any issues or further questions just ask and someone will reply as soon as they can.

Edit: Link for preparatory steps https://developers.google.com/android/images#bonito Make sure to revert to factory image PQ3B.190801.002 before continuing: