In QGIS 3.10 (Latest LTR) there are several Unit types, see image below
What is the core difference between those Units, in sense of their visualization behaviour on the Map?
What I noticed by so far, that some features do not always work with Millimeters, e.g.
Geometry Generator and therefore you somehow need to switch from one Unit to a different one.
On the other hand, you may work with a map, for example in Print composer, and furthermore, such Unit change can cause a huge impact on your visualization outputs.
Hense, I am looking for some kind of transforming strategy/approach that can lead me to the same visualization outcome but with the application of various Units.
Perhaps someone can refer me to any kind of explicit source with explanations.