Proper use of concepts like async/await for IO bound activity is probably the most important thing. There are very few tasks that are truly CPU bound that a typical user is doing all day. Even in the case of gaming you are often GPU bound. You need to fire up things like Factorio, Cities Skylines, etc., to max out a multicore CPU.
Even when I'm writing web backends I am not really thinking about how I can spread my workload across the cores. I just use the same async/await interface and let the compiler, runtime and scheduler figure the annoying shit out for me.
Task.WhenAll tends to be much more applicable than Parallel.ForEach. If the information your code is interacting with doesn't currently reside in the same physical host, use of the latter is almost certainly incorrect.