The navigator (left sidebar) is where you'll find your Screens, Components and a tree of all the Elements inside. It also gives you access to the Assets tab, where you'll find all your uploaded Media, Typography and other Style tokens.
Here you can find a list of all the Screens in your design. To create a new Screen, click:
Right-clicking a Screen brings up three options:
Here you can find a list of reusable Components in your design. To create a new Component, click: + New Component
Right-clicking a Component brings up the same options as for Screens (See above).