The toggle in the upper right of your note window will toggle between an editor mode and reading mode. The icon is the opposite for many of us as it indicates the mode to go to. So if the icon is the book, you are in the editor mode (and can switch to reading mode).
When in the editor mode, you can toggle between source mode and live preview using a toggle in the right click on the menu in the upper right. It’s all a little confusing in the start…
Finally, to go directly to either of the three modes , you can click the icon in the lower right and select which one you want to be in.