A question about upgrading on Debian/Ubuntu

I upgraded successfully on our test machine from V11 to V12 the other day.
Now it is time to upgrade the production machine. One f the lessons
learned in doing this is to do "make install" on all the extensions used
in the existing DB before doing the pg_dropcluster.

On the production machine I am confused by the state of the V12 server:

If I do sudo apt-get install postgresql-12 I get a message about this
package being marked to be "manually installed: pg_lsclusters reports on a
version 11 instance:

stan(at)smokey:~/src/pgemailaddr-master$ pg_lsclusters
Ver Cluster Port Status Owner Data directory Log file
11 main 5432 online postgres /var/lib/postgresql/11/main
/var/log/postgresql/postgresql-11-main.log

And when I do a "make install" on one of the extensions, it installs in
the V11 location.

What do I need to do to get the V12 server package correctly installed?

Thanks, in advance.
--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin