In this specific case, @Jakinta, you have final set restults that doesn’t overlap.
Region union is not good in this matter.
Replace 11 with 3 and 3 with 11 on first panels and you’ll understand.
You can do something similar but with meshes, see here: Sort intersected curves
Also, I believe @anon39580149 is searching for a pure math (with sets) solution, without going geometric.