You can find it in Preferences->KeyMap:
"Select next Tab in multi-editor file":

The keymap depends on the platform:
- On Mac: CRTL+SHIFT+RIGHT
- On windows/Linux it is ALT + Shift + Left/Right.
You can change it on the KeyMap panel.
You can find the buttons to switch between design, text and preview in top right bar:

With Android Studio Arctic Fox 2020.3 you can find the buttons to switch between code|split|design:
