Andersen Casement windows

This is kind of a follow up question or maybe a "I'm curious" question.
I just had some Andersen casement windows repaired by a local shop.
They had to order the glass. That tells me that they received a sealed double pane of glass (I assume from Andersen) to fit the sash. But I can't see any possible seam or "break" in the sash frame to install it. But after closer inspection I see these slots around the inside perimeter of the aluminum "bead".
Are these access holes to be used to remove and/or insert new glass?
If so, why can't I do that myself in the future?
Did an exhaustive search online to find some kind of instruction but to no avail.
FWIW I don't think these are gas filled, just a vacuum.

 

The glazing stop on most Andersen casements is on the outside.

 

So how is the ordered glass inserted into the frame or sash? I see no marks or scratches or nicks indicating anything was removed or taken apart. Even the sealing gasket of the sash to the frame looks as used and untouched as when I took it down from the frame.

 

From the outside. That's the way its supposed to look when it's done. Like new.