It's pretty amusing that they assembled it with a boring old construction crane. No fancy VAB or custom scaffolding. No different than the latest cookie cutter "luxury apartment building".
This crane is neither boring, nor old. It's an engineering marvel itself. It's a Liebherr LR 11350 [1], one of the most powerful mobile cranes available worldwide, modified by SpaceX for their own use case [2].
[1]: https://www.liebherr.com/en/int/products/mobile-and-crawler-...
If you deal in Javascript all day a tube frame and keeping your bearings from corroding sounds hard but all this stuff is trivially solve-able. Big complex machines like this are the equivalent of a tall stack of software. No one part is complex. It's basically a given that it will work. Whether it scales is somewhat up in the air.
The most impressive thing about it is that it can be economically viable to operate while complying with the litany of relevant regulations across multiple jurisdictions. If you just need to stack rockets it's not hard to come up with a cheaper and better one trick pony that only lifts rockets. You see examples of such one trick ponies in every shipyard.
But SpaceX is constrained to some degree by engineering man hours so they were glad to just write a check for an expensive generalist piece of equipment. Cranes like this didn't exist in the 1960s so NASA built a bunch of traditional gantry cranes into a building. SpaceX can skip that sort of fiddling around and just write a check, so they do.
Originally they hired a company that normally builds water towers to construct it. The idea is that the rocket needs to be a commodity, not something built in a clean room by specialists.
In a risky venture this is very hard to achieve.
SpaceX can just say 'we are gone do it this way' and they have almost everything required in house and can plan for mass manufacture from the beginning.