How the outline is made
Every clear pixel that touches the drawing takes the outline colour. With rounded corners only the pixels directly above, below and beside the drawing count, which leaves the diagonal corners open: the outline pixel artists draw by hand. Square corners count the diagonals too.
The outline is made of whole, solid pixels. A soft edge or a faint glow already in the image is kept and laid over the outline, so anti-aliased art gets a clean outline too.