After clicking the Editor menu option, the Full Site Editor (FSE) will appear. The screen will look similar to the Block Editor that you’ve been using, with some notable differences.
The left-hand Dashboard menu will not be displayed whilst in the editor, and unlike the Block Editor, when editing your Posts or Pages, there’s no option to make it display. Clicking the WordPress logo in the top-left corner of the screen will toggle the main navigation down the left-hand side of the page. This navigation will only display a few options that are used within the Full Site Editor itself.
To exit out of the Full Site Editor and return to the main WordPress Dashboard, click the < Dashboard link in the left-hand navigation.
The Templates menu option allows you to select which template to edit, along with providing the ability to create new templates. This latter functionality is very limited and incomplete at the current time.
Likewise, the Template Parts menu option allows you to select an individual template part to edit, such as the Header or Footer, along with creating new template parts.
The Site menu option returns you to the main Editor screen where you can edit your template.
Most of the tools/icons in the main Editor Toolbar at the top of the screen will be familiar to you if you’ve been using the Block Editor. There are a couple of notable exceptions though.
The dropdown menu in the centre of the Editor Toolbar () simply allows you to quickly navigate to a specific Template Part within the current Template that you’re editing. It also tells you which template you’re currently editing.
In the top-right of the screen you’ll find a Settings icon () which toggles the Settings Sidebar open and closed, just like in the Block Editor. Next to this is Styles icon (), which which toggles the Styles Sidebar open and closed. The Styles Sidebar is where you change the Colors, Typography settings and other options. This Styles Sidebar is what’s replacing the Customizer, for FSE Themes.