Current directory and all its subdirectories - to be exact :)

You can execute the find command only (with arguments, so until the pipe) to verify modified files beforehand.

If you are talking about the mkv embedded title, try this:

find -type f -iname "*.mkv" | while read "i" ; do mkvpropedit "${i}" --edit info --set "title=" ; done

As far as I can understand, even with EFCore in 10.11, there still will only sqlite be available as a database backend. There are plans for postgresql and other types but it's a much more distant prospect. Reference