That explains a lot, we were recently shown a video from way back then showing how they used layers then. It made all of us cringe in horror.
If you don’t change the active layer you don’t need to chase the active layer or wonder what face or edge is on what layer.
If you leave layer0 active and use entity info to assign a layer tag to groups, components, dimensions and other entities, you can manipulate their visibility without any qualms.
And the correct procedure is explained under the link? Then I’ll see it this evening and try to understand what is the difference to my way of working.
yes in fact, not putting initial geometry on layer zero has caused me a many of headaches while learning, when trying to turn layers on and off you can end up with these outline/selected ghost geometry that appear
When I had this problem, I activated all layers, ungrouped the geometry, and then grouped at the right layer. For me that was not a big deal. But I admit, working on Layer0 right now and assigning the right layer later is certainly more time-saving.
Edit:
Actually, I just wanted to know how to activate the levels in SU 2019. 
And I am learning a basic after almost 20 years of using Sketchup. Thank you all.