You just play them a lot to get used to and it's a lot less frustrating overall. People put a shit ton of time into MOBAs and the way the game works kind of adds to why people spend so much time playing them. You learn a lot about it by just playing, step by step in very small increments until you amassed hundreds of hours and grasp most of what there is to know.
When you think about it, learning a MOBA is extremely inefficient when you look at how much you learn and how much time you spend playing it, but execution isn't as difficult for the most part (an intense teamfight in a MOBA is not as difficult to execute as a combo in a fighting game in my opinion).>objectives and the time to tackle them
You just want to push whenever you can, this depends on the state of each player. Let's say you want to push a lane, you usually want to do it with teammates so if they can't (need to farm more, dead, doing something else), then that falls out of the question>positioning, remembering to ward, checking what's on the map
Those are pretty difficult and absent in lower level play but these things develop over time and watching guides, not as hard as you might think. Playing decent support requires more knowledge overall, but someone might ping you a position where exactly they want a ward for example.>what item to buy and when
Probably one of the worst aspects since there are so many items to buy. This is annoying but if you focus on one role, then you'll usually end up buying the same/similar items. This eases up learning item purchases and can be applied to other characters within the same role for the most part.>paying attention to my life/meter and checking on life/meter of other 9 players
That's a bit of a stretch, you don't actively have to pay attention to every health bar there is at the same time at every point of the game, but want to focus the one that's either high priority or low on HP.