For Atomic it might be worth looking into this first, as there's a fair bit of material out there already: https://pastebin.com/Bfq4jwsc
Tipau's youtube channel is the main place I would recommend anyone start if they're new. There are lots of other studies and resources listed there, but many are of somewhat dubious quality. Tipau's youtube videos are somewhat outdated in terms of opening theory, but they contain lots of very useful information and show how you should be thinking about atomic chess at a high level. Additionally, they explain some basic endings that I see lots of weaker players mess up frequently.
Hello guys; I am currently creating a Variant resource-like blog; And I am trying to cover as much Variants as possible; I have done the no castling part for now; I would love to have your comments!
https://www.chess.com/blog/1-c41-0