By default they are in the sub-directories at or below:
/var/lib/mysql
For example here is a listing of the wordpress database stored in MySQL/MariaDB
[root@lampcent7 wordpress]# pwd
/var/lib/mysql/wordpress
[root@lampcent7 wordpress]# ls -altr
total 168
-rw-rw—-. 1 mysql mysql 65 Jan 18 10:57 db.opt
drwxr-xr-x. 5 mysql mysql 182 Jan 18 15:25 ..
-rw-rw—-. 1 mysql mysql 13064 Jan 18 15:34 wp_users.frm
-rw-rw—-. 1 mysql mysql 8684 Jan 18 15:34 wp_usermeta.frm
-rw-rw—-. 1 mysql mysql 8682 Jan 18 15:34 wp_termmeta.frm
-rw-rw—-. 1 mysql mysql 8668 Jan 18 15:34 wp_terms.frm
-rw-rw—-. 1 mysql mysql 8768 Jan 18 15:34 wp_term_taxonomy.frm
-rw-rw—-. 1 mysql mysql 8666 Jan 18 15:34 wp_term_relationships.frm
-rw-rw—-. 1 mysql mysql 8688 Jan 18 15:34 wp_commentmeta.frm
-rw-rw—-. 1 mysql mysql 13380 Jan 18 15:34 wp_comments.frm
-rw-rw—-. 1 mysql mysql 13176 Jan 18 15:34 wp_links.frm
-rw-rw—-. 1 mysql mysql 8698 Jan 18 15:34 wp_options.frm
-rw-rw—-. 1 mysql mysql 8682 Jan 18 15:34 wp_postmeta.frm
-rw-rw—-. 1 mysql mysql 13684 Jan 18 15:34 wp_posts.frm
drwx——. 2 mysql mysql 4096 Jan 18 15:34 .
Each file contains a table of the same name minus the extension.
File location can be customized by changing the line that follows from /etc/my.cnf
datadir=/var/lib/mysql