Many iPad keyboards have a dedicated button to show the on-screen software keyboard, which can be handy if you want emoji or a third-party keyboard for things like text snippets, form filling tools, and GIFs.
But if your keyboard doesn’t have such a button, or you just prefer touchscreen controls, you can quickly show the on-screen keyboard by double tapping in any text field. The keyboard will slide up so you can do your thing. When you’re done, tap any hardware key to dismiss the on-screen keyboard and get back to typing.
I think this works on iPhone as well, I just haven’t tested.