In iPadOS and iOS 13, you can kinda replace the stock Apple apps on your iPhone or iPad. For instance, anytime you tap the Mail icon, Spark could launch instead. Or, and this is probably the most useful, when you tap the lock-screen shortcut for the Camera app, you could launch Halide instead.
This isn’t proper app replacement, but it is a pretty wild trick for iOS devices. We use a new feature in Shortcuts to make the magic happen. And for the Camera app, it works great.