Scotty here, the dude that made the video. I'm not sure I can answer the first set of questions about why phones are fragile. I think it probably has something to do with consumer expectations/tastes around look and size of phones though.
But I can give you some answers for your second set of questions.
> So what does all that Shentzen parts retail look like when you're building something new?
The markets originally started as wholesale components and tools markets to service the electronics factories in Shenzhen, which make a significant fraction of the worlds electronics, particularly commodity electronics. So it actually mostly caters to people building new things, versus repair. But I didn't show any of that in the video. My understanding is that the cell phone repair markets are comparatively much newer, and obviously cater much less to large scale production of new goods.
> Can you go into those buildings with a bill of materials and come out ready to load your pick and place machine?
Yes, absolutely. There are multiple buildings of distributers that do nothing but sell components on reels to load in pick and place machines. You can even buy a pick and place machine there if you need.
> Can you get boards fabbed one-off while you wait?
Not quite while you wait. There are a bunch of booths in the markets advertising these sorts of services though. I think the fastest on the PCB fab itself (without assembling any components on it) is 12-hour turn time, and that's pushing it a bit. 24-hour turn time expedited but pretty standard, and 3-4 days is generally the non-expedited standard for the board house I've used. Sometimes a bit longer if they're really busy. A courier on a motorcycle will bring it to your door.
> Or is it all about repair and simple mods?
The cell phone markets are mainly about repair/refurbishing and simple mods, as far as I can tell. But I don't fully know - there's still a ton more for me to learn. I still feel like I'm barely scratching the surface here.