Hi @atarilinux
Thanks for your enthusiasm.
To start, here is the porting documentation because you'll probably need to prepare the development environment to build your own image for the Pixel3a.
https://docs.ubports.com/en/latest/porting/introduction/index.html
@fredldotme might find some time to give you more insights on how to proceed and the exact repository for the pixel3a.
As you see we're not connected all the time, but I try to keep up with the forum as much as I can.