On an empty scene you will see the left menu, but if you right-click a visible source in the preview area, you will see the menu on the right:
Allows you to start a fullscreen "preview/projection" of your current scene on a different Monitor.
Allows you to start a fullscreen "preview" of your current scene on the current Monitor the OBS window is on. You can still edit your scene and view the control panel if wanted.
Scale to fit means your scene will be scaled to fit the size of your preview, while in 1:1 mode, OBS ignores the preview Area size and uses your Base resolution to render the scene at its original size.
Enable or disable the preview.
Enable or disable the bottom Control Panel area.
All usual source options are available. (Add, order, size, position, etc.)