And heaven help you if you want to find out where all of that data is on the hard drive.
Oh, by the way, it's at:
Apple menu -> About This Mac -> Storage
And don't alt-tab away from it; you can't get back to it using the keyboard. You have to use Mission Control or back through the Apple menu again.
While it works on folders, it just gives you a question mark for the size while it is working out the size of the sub-items on disk with no indication that it's actually going to change that.
So yes, there are alternative methods, but they are no more friendly to discoverability as the Apple Menu method.
The answer lies in your question. A lot of power user (yes, relative but Apple have focused on consumer) functionality is buried in the gui, and every release pushes stuff deeper in to the interface.
"What's the command that shows me file system usage again?"
> man -k file system usage
result = 5 pages of matching commands, often poorly formatted, word wrapping oddly, sometimes without even carriage returns between the commands, so it's all one long list. And this is in Unix which has had something like 45 years to clean up this mess.
I don't hold out hope that this sort of interface will ever be presented usefully, and we already have a better system in a GUI.