Electrical – Altium: What are these vertical yellow lines

Those are error markers (they indicate that your nets are not connected). Go to the Menu, select "Reset Error Markers", I think it's under design or Tools.

Hopefully I understood what you are trying to do.

You have to create a mechanical pair.

With a PCB open click Design Board Layers and Colors (Shortcut: L)

In the bottom left corner, you'll see a Layer Pairs Button. Click it.

Add your layer pairs together (like Mechanical 14 and Mechanical 30).

That's it.

Now when you switch components between layers, the items on the mechanical 14 will switch to mechanical 30 and vice versa.

I'll try and add some images and format this answer to be a bit more clear later on.