Hello! It all depends on how you want to develop. I now know designers who became great video editors, try using good textbooks and good teachers. Also, use the right software right away.
There are several options that I could offer, but they are similar. Download either the converter, which has many formats for conversion, or the editor (I use https://videoeditor.com/, which I advise you too). Try converting the files or changing them, the main thing is that the application itself determines the format and reads the data, and then you can just save it in a form convenient for you.