2 possibilities that ultimately call for the same function but whose behavior will not be the same despite everything.
Either when inserting you check the box "with the move/copy function", or you use this function via insert/functions/"move/copy". This function can be used in 2 ways: either via translations/rotations along defined axes or via constraints close to those available in assembly. Note that when you combine insertion and function, SW doesn't like it when reordering the functions created later.
Question: what is the point in your case of inserting the 2nd part into the first one rather than making an assembly?
I want to join them together with 2 existing independent 3D parts, as if welded to each other by the Bossage/Extruded function and then transform into a sketch of a single part that I could modify.
If it's just for these 2 pieces that the problem arises, given the non-complexity, it's faster to measure and remodel them correctly, rather than making convoluted constructions with links between files.
No, not all the answers have changed status, it's just the display that is changed to allow you to change your mind. The only real answer is the one outlined in green just below your question.