Looks like there are no standard images available, so you’ll have to follow the standard pmbootstrap installation guide (https://wiki.postmarketos.org/wiki/Installation/Using_pmbootstrap). Can’t say how well the device is supported, either it’s not very well supported or the wiki page never got updated.
For your convenience: https://wiki.postmarketos.org/wiki/Google_Pixel_3a_XL_(google-bonito)
Looks like there are no standard images available, so you’ll have to follow the standard pmbootstrap installation guide (https://wiki.postmarketos.org/wiki/Installation/Using_pmbootstrap). Can’t say how well the device is supported, either it’s not very well supported or the wiki page never got updated.