I started at the beginning of Season 22 and have mostly been playing Solo. It took me about a full season to learn about the synergy between certain items (I'm still finding synergistic combinations that weren't obvious at first). It took me this last full season to better understand some of the game mechanics and how things are calculated.

Icy-Veins is a bad place to find builds but honestly I think they explain the synergy between certain items very well.

Maxroll.gg is all around the best source of information and their articles on game mechanics are essential, you should definitely read them. If you join their Discord, they are very active and will reply if you ask them a question.

Cratic is probably the best theorycrafter I have found. His website is down at the moment but his video guides are very detailed and they link a spreadsheet with all his builds from the same patch. Last season(2.7.0) I had alot of fun with his 2.6.9 Poison Scythe Necro build and learned a lot from his videos specifically.

Take your time and have fun with it. I suggest finding a class or two that you like and learning to play around their Convention of Elements cycle. Necro and Wiz are my favorite and I'm still having alot of fun playing and theorycrafting builds for them.