First of all, Big Sur on the Apple Silicon allows to run iOS apps unmodified, so that requires no work, and secondly, the UI redesign of Big Sur means, that iOS

First of all, Big Sur on the Apple Silicon allows to run iOS apps unmodified, so that requires no work, and secondly, the UI redesign of Big Sur means, that iOS/iPadOS apps are easier to port, as the basic look and feel is the same.