How to use grep to search only in a specific file types?

You don't give grep a filetype, just a list of files. Your shell can expand a pattern to give grep the correct list of files, though:

$ grep MYVAR *.yml

If your .yml files aren't all in one directory, it may be easier to up the ante and use find:

$ find -name '*.yml' -exec grep MYVAR {} \+

This will find, from the current directory and recursively deeper, any files ending with .yml. It then substitutes that list of files into the pair of braces {}. The trailing \+ is just a special find delimiter to say the -exec switch has finished. The result is matching a list of files and handing them to grep.

find . -name \*.yml -exec grep -Hn MYVAR {} \;

If all your .yml files are in one directory, then cd to that directory, and then ...

grep MYWAR *.yml

If all your .yml files are in multiple directories, then cd to the top of those directories, and then ...

grep MYWAR `find . -name \*.yml`

If you don't know the top of those directories where your .yml files are located and want to search the whole system ...

grep MYWAR `find / -name \*.yml`

The last option may require root privileges to read through all directories.

The ` character above is the one that is located along with the ~ key on the keyboard.