Does the app get told when its rider has picked up the order?

For some apps, yes. Once an order is marked ready, a Picked up button appears beside it on the Delivery apps page. For Talabat, foodpanda and Keeta pressing it is sent to the app. Deliveroo, Careem and GrabFood have their own rider apps that report the collection, so there is nothing for us to send and the message on screen says so. The Orders from delivery apps tab in the docs lists what each app accepts.