I will attempt to answer the first two aspects of the question. My source is Gordon, J. E. (1981). Structures, or, Why things don't fall down. Gordon's Structures is a very easily readable, and funny, technical book about structures.

In this book Gordon discusses the development of masonry and why medieval masons could build cathedrals without anything like a modern engineer's understanding of or ability to model structural forces. (He specifies mediaeval, presumably because we have such gaps in our surviving knowledge from Roman or earlier times that it's possible their structural mathematics was much more advanced.)

In a building, generally the main force acting on it is gravity, which consistently acts downwards, so objects subject to gravity are said to be "in compression". This contrasts to forces that act to pull a material apart (like you pulling the ends of a rubber band or a ship moving through heavy waves), where the material is under tension.

Masonry materials (eg bricks and mortar, and concrete) are very strong in terms of compression. It takes a lot of weight to crush a brick. Therefore, if you build a small building and it stands, and you then build a bigger one to the same design it is likely to still keep standing because although it's heavier, the forces within it are still well within the strength of the brick. Gordon mentions there is evidence that mediaeval masons would build large scale (many feet wide) models.

This compares to say ships, which are subject to much more complicated forces (and also need to keep floating) and thus are much more demanding to design and build at large scale.

There are limitations to the " scale up", eg if the ground subsides beneath the weight of the building, or worse, part of the ground subsides and another part doesn't. And the sideways force of the roof can be an issue. And, when it comes to very tall buildings like skyscrapers, the effect of the wind across an entire tall building introduces large sideways forces that a smaller model doesn't have to cope with. There were some cathedral failures.

But, I hope this answer indicates how Ancient Romans could build large buildings without modern structural science and why they can still be standing.

(As an aside, Gordon in his book also speaks a bit about how the Romans built powerful seige machinery, he thinks very highly of their prowess there. It's a fascinating book and I highly recommend it even if you find the general topic utterly boring.)