14 - QGIS Tutorial - Fixing Messy Boundaries Using Difference 

Using QGIS 3.14, this shows a workflow for fixing mismatched boundaries when drawing from multiple vector boundary datasets of differing accuracy or resolution.
Uses dissolved prefecture boundaries from 1911 v6 China HGIS data:
chgis.fas.harvard.edu/
Uses country data from Natural Earth Data:
www.naturalearthdata.com/