re: "How does the client get the app when the developers is finished?"
Upwork is a general-purpose resource for hiring online freelancers. It is not an app development site.
Thus, your question is not really an Upwork question. It is a question best handled between you and the freelancers you hire.
If you are quite new to hiring freelancers to develop an app for you, then you need to work with an independent project manager. Period.
You are not qualified to serve as the project manager yourself. Upwork does not serve as the project manager.
There is a 20% change that the lead developer you hired CAN serve as his own project manager.
Are willing to take an 80% risk of project failure?
You need a project manager who will advise you on how to take delivery of the app.
Here's a hint:
NOT "when the developer is finished"