Your 3D model can be one single element or the result of a group of elements.
If you want to combine two objects belonging to two different groups, be careful because those two won’t cross correctly.
SketchUp can create groups of objects: right-click > Groups.
See how the entities have changed their color:
If you realize you need the objects divided, you can do it, you can “Explode” the entity.
MeshMixer works kinda the same way. Import 2 entities inside the software, like we did in the past. You’ll see them on the screen and the Object Browser will pop up. Notice that the two entities are on different levels, you can see it in the Object Browser.
Select both of them, then Edit > Combine.
Separate Shell deletes the groups, just like the Explode function in SketchUp.
Inside the lessons 54. and 5.3 you’ll find the tutorial about how to move inside the groups an subgroups.
Sometimes you can even separate the object inside the slicing software, like Cura 3D.
import the entity and right-click on it > Split object into parts. The two objects are divided, You can also move them.