Framing material in the US is called SPF, which means spruce pine fir. Any of those species may be used and they have similar enough engineering characteristics that they are interchangeable. Occasionally a building will be designed to use a specific species (southern yellow pine etc). But mostly we engineer stick built structures for SPF framing.

Fir is used in the US (most commonly in the pacific northwest), it's just typically used for outdoor construction because it doesn't warp as easily and holds up far better outdoors than pine. So you will frequently see it used as decking, fences, or siding (although wood siding is getting pretty uncommon).

Fir is also sometimes used for framing like pine is but pine is usually cheaper. The only advantage fir has over pine for framing is that it tends to be a bit stronger. But there are very few places where you need that extra material strength and can't just use slightly more reinforced pine construction to acheive the same result.

The "new growth" part is because the trees are harvested from farms as soon as they're big enough. This is as opposed to "old growth" where the trees are more mature and stronger.

Think veal versus beef in terms of texture

is clearly definitely not a beaver

is clearly not trying to determine where to avoid shitty soft woods