Where Is MySQL installed on My Machine?

Debian versions of MySQL packages store the MySQL data in /var/lib/mysql directory by default. You can see this in /etc/mysql/my.cnf file also.

Debian packages don't contain any source code, if that is what you meant by source files. Binaries are installed generally in /usr/bin and /usr/sbin directories.

You can see where the package files are installed by using dpkg -L <packagename> command. Note the capital L letter.

If you want to know the packet name for MySQL, you can use dpkg -l | grep mysql to view all packages that have mysql in their names.