I've dismantled a padlock before, different type, but it still worked. The one I dismantled I had to use a heat gun to expand the casing and a heat sink to keep the interior cool.
You can actually assemble things inside a bottle using callipers and dividers, they generally have much better length on them than a pair of tweezers and are much easier to pick up than an adequate pair of forceps. I wouldn't doubt some of the narrow necked items could be assembled using forceps.
Easy, the padlock used is a master lock type. By removing the bar from the lock (which easily fits through the neck in any direction) you can insert the mechanism half of the lock sideways (where it is narrow enough to fit through) and then you reassemble the lock inside.
One obvious method would be to make the bottle in two halves and join them together with the contents in place. Either split it down the middle or -- this might be easier to do without it being noticeable and without damaging the contents when joining the pieces - make everything apart from the bottom of the bottle in one piece and attach that at the end.
Whether that's actually what they do, I don't know.
The bottle with the corkscrew on the inside is my favorite. My best guess is that the corkscrew handle, without the screw, was hung inside by threads. The cork-with-screw was then pressed into place, probably with some glue on the end of the screw. The handle could then be pulled onto the screw with the threads, and the threads pulled out of the bottle.