On YouTube creator Jenn Jager has started an Intro to Pixelmator Pro series that you may find helpful:
1. Quickstart for Beginners https://youtu.be/cFo56s2rP6U
2. CLIPPING MASKS AND BLEND MODES https://youtu.be/jGbPOwbHCls
3. Fix Images with Repair and Clone! https://youtu.be/4jbBRGoUjbE
4. Introducing 3.3 Mosaic https://youtu.be/lfko17pyUeg