I wonder what stage 5 actually is then? We've been to the Cali factory in Hannover (and the main T6 plant) but it was back in 2018 and my memory isn't good enough to remember if they explained the different stages to us specifically. You can see from below that there is a lot that happens between status 20 and 30 - so stage 5 could be something like components for the kitchen maybe, or the roof?
In a nutshell it goes like this (I don't know about the exact process for the beach, apologies);
(start of status 20)
Transporter factory (site 1)
stamp panels, create body shell (with hole pre-cut in roof for cali and side panels with holes already in for water/electric etc)
create running gear / engine/gearbox etc
paint bodyshell
'marriage' of engine/gearbox etc to bodyshell
some interior trim fitted (dashboard, steering wheel etc etc)
basic tests
transport unfinished van to site 2, cali factory on the other side of Hannover (3x vans transported at a time, on a car transporter, by road, covered trailer - as the unfinished vans just had a hole in the roof at this point)
Cali factory (site 2)
floor fitting
kitchen fitting
interior fitting inc seats etc
roof fitted (they have stock of all colours of roof panel and bellows and make them up to spec manually for each vehicle going through. bellows are manually fitted to roof)
Each cali is then tested for water ingress - they lift the roof in one of two special water test bays where they simulate heavy rain from multiple angles, while someone is inside checking for leaks around the roof/bellows etc
Cali leaves factory, gets moved to Emden on a train from Hannover. (status 30)