It's really easy:
Just tap & hold with any brush on any closed contour
Or pick the Fill tool and tap
⚠️ Mind that this will outline all surrounding strokes into a single vector shape: all path strokes will be merged as a single outlined shape, thus loosing the ability to later edit the path skeleton of each merged stroke.
Selecting the Pen tool
Tap the Brush tool icon to view other drawing tools
Tap the Pen tool icon
Preview fill area
To preview what shapes the Fill tool will merge, tap the Fill tool icon & move the pencil over the closed shapes to preview the resulting outline
(This requires an Apple Pencil 2 or Pro)