Click on file to see mediainfo.

The mkv contains PGS subs.  If you want to use the FFF subs instead, one way is to have them in the same folder with the same file names as the mkv files.  Then most players will default to use them or at least list the external files as an option.

If you mean "put in" as permanently insert them in the mkv, you'll need to learn how to use software such as mkvtoolnix.  Google that, if you're interested.

The subtitle file is in another folder called "Love Live! Sunshine!! (FFF) Subtitles,  same goes for Season 2, the movie is fine though, so I want to know how to place the subs in the episodes so I can start binging it, Thank you very much for your help.. I really need it.