Making sure your scenes behave as you expect can be a complex subject, but those videos provide a starting point and explain some of the tools that Polygonjs gives you to help on that front.
1 - Use The Node Info Popup
This popup helps you quickly see what geometry is produced or modified by a specific node.
2 - Use The Spreadsheet
This shows you in details all the attributes that make up a geometry. If a position or normal value was to be different than expected, you can then spot it very quickly
3 - Use The Performance Monitor
This helps you see how long each node needs to do its work, so that if one was too slow for your needs, you can either change the order of operations or use a different method.