A lot of that stuff you pretty much just have to experiment with and, as Lyrica said, learn the encounters. When a big hit is coming (on the tank or on the raid), you have a variety of tools that can greatly reduce the incoming damage. These tools depend on whether you want raid-wide reduction or single target, physical or magical or both. Some have things like RNG dependence.

Perhaps look up a few SCH PoV videos for encounters you want to do and check what they're doing. If you have any specific questions on why they use a certain ability at a certain time in that encounter, then come back here and ask about it.

And thoroughly read your tooltips so you know what everything does.