General help

Running from localhost would require changes to config.php.

DB server should be 'localhost' - but DB user/password is probably different.   So would wwwroot from FQDN to localhost.  And maybe path to moodledata.

Not only that, but all internal links in the DB had https://olddomain/ in them.  You must now convert all internal links to http://localhost/ assuming that one can no longer run a valid ssl cert for your localhost computer.

You've not mentioned what operating system this 'localhost' is?  Windows/Linux (what distro).

As I have not used Windows in many years, the following is linux:

From terminal, ps aux |grep mysqld

to see if mariaDB server is running.

If it is, then again from terminal, what happens when you attempt connection to the DB using the mysql client application?

mysql -u userseeninconfig.php -p'passwordseeinconfig.php'

'SoS', Ken