1. There are two editors, the markdown editor and the rich text editor, you can switch between them with the buttons on the top right (M icon is the markdown editor, other one is the rich text editor)
2. The markdown editor has three views (editor only, viewer only, split view), you can switch between them with the toggle editor layout button:
The layouts that it toggles through (editor only, viewer only, split view) when using "Toggle editor layout" can be changed under View | Layout button sequence:
If you don't like/don't use the split view, you can set it to "Editor/Viewer" as in the above screenshot so that it only toggles between the editor only ("plain text") and viewer only modes.
Note that on mobile it works differently. It always defaults to "Reading" mode when accessing a note, and clicking the edit button will open the markdown editor by default (and there are no separate markdown editor views on mobile). However, you can switch to the rich text editor by clicking the hamburger menu on the top right | Edit as Rich Text. IIRC on mobile it will remember your last used editor so next time you click the edit button it will open that editor.