Shared bikes - what makes them robust?
Mass - resists denting and damage.  Some are steel, some are aluminium.
Single-speed - a lot of city bikes only have one gear, and those that have more are often euro-IGH (internally geared hub) with 3 speed, no more.  Either way they have 1/8" singlespeed chains.
Chainguards - to keep the chain and pants clean.
Robust lubricants - intended to not wash out.
Tyres - fairly hefty, large, and puncture resistant means fewer flats.


And the big one:


Ongoing maintenance  You may not see it, but the company will do ongoing maint of their bike fleet.  Most of the apps support some kind of "report a problem" so that the fleet staff know where to go to find and fix the bike.  Those that don't have etiquette


Cultural factors will come into it as well - suspect there are more vandalistic yobs in the west than in China.