Widgets are the fundamental building blocks of a website. Widgets can be as small as a button or as big as a section containing a contact form.
Click on the Plus button on the left.
Drag the component to the place where you would like to place it.

Optionally, you can also click on the component directly to place it right at the focused element on the page.

There are some widgets that have a premium badge on top. These widgets are not available to free plan users.
Widget can potentially contain other widgets and offer a set of configurations for you to customize. To see how to use each widget, check the help articles of each widget.
The widget selector also allows you to collect widgets from different web pages and reuse them later.

To collect, just click on the "folder" icon on the toolbelt and follow the wizard. Or just click on the "copy" icon to save it temporarily.