Trying to get more information on video formats and how they work for media player project

I'm trying to start getting into some more intensive programming projects on my own time to practice my programming and try and build up somewhat of a resume for myself. Anyways I want to create a media player completely from scratch something like VLC player but not as intense. Anyways i'm trying to find any information that would help me out. UI and graphics for this I think I have down and could handle but the actual opening of the media files, and playing them, and how they work, and in general how I would go about creating a programming like this I need some help with.

You'll want to Google "filetype specification". For example: here's the spec for an mp4.

Video encoding is everywhere, and if you master it you can probably sit pretty writing code for any online streaming service (Twitch, Netflix, Youtube, etc).