Thanks to RokX's comment in the second answer, I was able to figure this out:
(I'm on a Mac) I realized the Code, Split, and Design options are at the top of the code editor window (which might have the title "MainActivity.kt" at the top left if you've just starting a new project template), not just generally in the Studio app. To anyone else, to clarify: look for the window in the app that shows code, and to the top right of that window should be buttons that will say Code, Split, & Design when you hover over them.