Not sure, but I can tell you for sure it wasn't welded after assembly.
Someone put it together , so it must come apart.
I would try removing the sliding glass first, then see if the fixed glass can slide towards the middle of the housing then removed. You might have to flex the extrusions apart abit.
Can't be difficult to figure out.
As far as seals, you may have to use one of the many different profiles available from Steele rubber or Soff Seal.
Edit;
Try here. Lots of profiles to chose from;
https://www.steelerubber.com/camper-rv/windows