Search through this Forum for us.yaml brought me here: https://world.pages.gitlab.gnome.org/Phosh/squeekboard/tutorial.html. Another one points to a work in progress (contributions welcomed, I’d guess): https://source.puri.sm/Librem5/squeekboard/-/tree/master/data/keyboards, etc.
There are no dead keys in Squeekboard. The closest to dead keys is switching the layout: the first press does not emit a character. The concept of dead keys doesn’t have advantages over layouts when labels can be modified.
Coming back to system layouts, not all of them are supported in Squeekboard. If one isn’t, then Squeekboard will fall to the closest appriopriate one for the given language. “US International” falls back to US. You could submit a new layout for that, although you’d have to explain the chosen scope cause I surely don’t get what the “international” is meant to be used for.