proposal: Postgresql 18 Linux (all flavors) - with installation, create new instance/main database: sudo -u postgres /usr/pgsql-18/bin/pg_ctl -D /var/lib/pgsql/18/data/ initdb
No, make/ninja install should not create a database. That is outside its appropriate responsibilities. Installing from source stops once the binaries are installed. Using the installed product is up to the user.
Also there are lots of reasons to install Postgres without creating an instance. This is actually one of the things I don't like about the Ubuntu installer, which defaults to creating an instance (although this behaviour can be suppressed by creating a config file with an appropriate directive first).
