A good start is to read the introduction and chapter 1 of the magnificent book by Ton de Leeuw - Music of the twentieth century : a study of its elements and structure

https://hanze.on.worldcat.org/oclc/62872243

Mindmap of the first chapter.

Playlist on Spotify

Slides (Powerpoint)

It's a free ebook available within hanze.nl :-)

I made a list of available ebooks concerning 20th century music within hanze.nl:

https://hanze.on.worldcat.org:443/list/16749714