The Layout & Content tab is the first one you are presented with when accessing the Experience Manager.
You can use the variation drop-down to navigate between variations. When a variation is selected, you will see a preview of it.
Clicking EDIT DESIGN will open a new tab and bring you into the page builder. If there are multiple variations within the page experience, you will be directed to the variation that was selected within the Experience Manager.
Clicking COMMENT will open a new tab and bring you straight into the preview, with comment mode toggled on, on the variation that was selected within the Experience Manager. The collaboration feature is Enterprise-only and if you're a non-enterprise customer you will see a modal pop up instead:
Please see this article for the next tab of the Experience Manager: Integrations