One thing I can not recommend enough is
The Balance if you are new. (
https://thebalanceffxiv.com/ - has the discord, and a ton of resources)
They are constantly theory crafting stuff, and will help anyone new to the game all the way to BiS for raiding. The discord has a ton of guides on proper rotations, melds, brackets for stats, when to use moves effectively, etc. Even if they don't have what you are looking for, they have a questions section for each class where a good chunk of people are willing to help out.
Honestly, The Balance is probably the thing I see brought up the most when asking for help, and it's probably the best way to get an up-to-date answer for the game.