Also takes 2 seconds... You don't need 3rd party apps like everyone's saying, only if you want tiling or to copy Windows behavior.
Press Control-Up Arrow (or swipe up with three or four fingers) to enter Mission Control, drag a window from Mission Control onto the thumbnail of the full-screen app in the Spaces bar, then click the Split View thumbnail. You can also drag an app thumbnail onto another in the Spaces bar.
I feel like anyone reading that, and thinking that is a reasonable/intuitive design, may be quite far down the rabbit-hole.
It reads like a parody.
It’s significantly worse than I even imagined.
It's two gestures, a swipe and a click and drag.
I'm not even saying Mac is superior here, just that there's a quick way to do full screen splits
So the trick is five hidden things, not presented in the UI. Great!