The official PostGIS code repository is a git repository hosted on OSGeo Gitea instance (https://git.osgeo.org/gitea/postgis/postgis).
To get the source from main repo, and start a configure:
git clone https://git.osgeo.org/gitea/postgis/postgis.git cd postgis ./autogen.sh ./configure
If you would like to submit pull requests via the main repo, you will first need an OSGeo LDAP ID
A number of CodeMirrors do exist for users and contributor's convenience.