Bridge Edges
The Bridge Edges action creates a new face between two selected edges.
By default, this action can only bridge two open edges (that is, no face on the open or free side). However, you can override this; to do so, navigate to the Preferences and enable Allow non-manifold actions.
You can use this action with the Alt/Opt+B shortcut, or from the ProBuilder menu (Tools > ProBuilder > Geometry > Bridge Edges).