Presumably you'd use UIViewRepresentable to wrap the SKView. The code to deal with the nodes would depend on the details of your application.

What's wrong with the code you've commented out? Though I think the type ought to be Task.Handle<…>.

There are only 32 bits in the bitmask. These bits are for allowing types of contact, or contact between types of nodes. You should use just one of the bits for all these balls. The contact test delegate will get called for each pair of nodes. In the contact test delegate, examine the ball node and make the changes you need.

Change the gravity in physicsWorld.

Traditionally, game developers have used an in-game console.