Widgets can be added to containers in the "Edit page" view.
Go to "Edit page" view
Add a container if needed
Click "Add widget"
Choose the widget you want to add
Edit the widget and save the changes
This is what the add widget modal looks like:
From top to bottom, you see:
A search bar: you can search for widgets using their names, keywords, or the description of the widget. You can search in any workspace language - not just the one you're using at the moment.
Quick Access: this is a quick way to add commonly used widgets
Widgets divided to categories: you can also just scroll down and see all the available widgets, including descriptions that tell you what you could use each widget for.