One of the benefits of the new Stage Manager feature is that iPhone apps makes a lot more sense on an iPad. You can even use them properly in landscape now, which is a pretty decent experience, as it turns out. The iPhone apps are resizable too, although not as freely as iPad apps.
Below are two comparison photos where Instagram (which is iPhone only, unless you do this) is in its smaller, normal, state, and stretched to the height of the screen. The screenshots are taken on an iPad Pro 11”, so you might get a little more zoom, because that’s what it is, on the 12.9” model.