I'm building an app for iOS/Windows. I want to allow my users to register / create a new sub site on my domain via the app. How can I achieve this? Does the REST API allow this?
There is nothing in the core or in the rest API for this requirement. You can try this plugin, https://github.com/remkade/multisite-json-api
Sorry, but I see no other chance and no more to say in this answer.
The core function to create a new site in the network of the installation is
wpmu_create_blog. But there is no access from the REST API.