Best sources to study

So, I have been wanting to formalize my training for a while now, trying to make some time for it, and with that purpose in mind I wanted to find out which is the best source one could get to study Opening Theory.

Is it books? Videos from a website? Where do professional players study their openings?

Would appreciate any comments.

You are obviously already advanced- with a 2051 blitz rating and a score of 40 on Puzzle Rush.

I find it hard to believe that you haven't already read many books, watched many videos, and perhaps even worked with multiple chess coaches.

Why don't you describe how you got to where you are currently?

Some professionals use...

ChessPublishing.com...

https://www.chesspublishing.com/content/

or one of the chess databases, such as Chessbase Opening Encyclopaedia and/or Power Book...

https://shop.chessbase.com/en/categories/chessbase

or, perhaps, New In Chess Yearbook...

https://www.newinchess.com/new-in-chess-yearbook

Improving chess amateurs might want to check out...

Chess Openings Resources for Beginners and Beyond...

https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond

ChessBase is a good investment if you're willing to spend that much money. I find its Live Book especially useful, which, for one thing, allows easy access to 7-man tablebases without draining your battery or disk space.

https://chessmood.com/courses The best place for improvement! 

chessme.io worked for me

try openings101, here you can find in-depth analysis for many opening lines. It's free and no need to register.