What language and data structures does MYSQL uses as backend?
MySQL can be compiled in C/C++ and you can download the source code to do so.

At this link, Under the Select Platform ComboBox, Select Source Code and Download it from there

The .frm , .MYD, and .MYI make up a MyISAM table

For example, the user table for the mysql grants is amde up of three files


/var/lib/mysql/mysql/user.frm (Table Structure for any Storage Engine)
/var/lib/mysql/mysql/user.MYD (MyISAM Table Data)
/var/lib/mysql/mysql/user.MYI (MyISAM Table Indexes)