See my notes on music here (in Armenian, since anyway there are many English sources out there).

Arrangements of some music on the piano

Here are several musicians I recommend to follow if you are into music and piano in particular.

Frank Tedesco is the ultimate piano master, possessing mind-blowing technique, over-trained musical ear and unimaginable memory. He has a series of videos where he asks random strangers to pick songs and instantly reproduces the music on the piano with the greatest accuracy, beautifying the song even further. Also in his secondary channel he creates the craziest loops on the fly with full orchestration.
Charles Cornell is a jazzman youtuber who analyzes game and movie soundtracks deeply and in detail. His videos are both entertaining and educational.
Andrew Wrangell is a composer and pianist of highest rank. Many tracks of Nightwish and Two Steps From Hell are arranged and performed by him. He is the co-creator of the channel Sheet Music Boss which releases a new piano tutorial every day.
Mikhail Tovmasyan is a pianist who arranges and performs various music on the piano.
Diego Ares is a virtuoso harpsichordist performing the classics of Baroque.

The music theory iceberg sums up all the material in the field.
musictheory.net offers basic lessons in music theory for starters.
Beyond Music Theory is a very good book for self-studying music theory, covering sufficiently great deal of material.
The Chopin Method is a YouTube channel that breaks down piano technique as it saw Chopin.
tonedear.com offers various exercises for ear training.
In ChordU one can find the chords of many mainstream songs.