workers used hot air gun to melt PVC filament (about AWG 8 ) to weld PVC toilet pipes.
PVC (polyvinyl chloride), as well as most other vinyl types, are thermoplastics. So are polypropylene and most polyethylene types.
All thermoplastics can be melted and then shaped, without affecting their properties when solidified/cooled down. If all pieces are of the same thermoplastic, they can be welded together with heat. If they are different, then their chemical makeup comes into play: if compatible, they will form a chemical bond, joining better than with adhesives. If incompatible, it'll only be a mechanical bond, like flexible caulking or non-adhesive weather stripping, that just solidifies into the desired shape to keep things in place. (Silicone caulking does actually chemically bond to many things, though, so it does have adhesive properties. Some adhesives penetrate a bit into the material, making a better bond.)