Puro has two widget areas, Sidebar and Footer. Add widgets at AppearanceWidgets.
Sidebar
The sidebar shows on the blog page, archive pages, search results and single posts. It also shows on pages that use the Default or Default Template – No Title template. It shows only when it has at least one widget. Without widgets, the content uses the full width. To move the sidebar to the left, use Theme Design: Sidebar in Puro Premium.
Footer
The footer splits into equal columns, one for each widget. Two widgets make two columns. Puro supports up to 10 columns.
Puro turns off the block widgets editor, so Widgets shows the classic screen. To use the block widgets editor, return false from the puro_disable_new_widget_area filter. For a rich text widget, see Rich Text Editor Widget.