I'm working with an agency and they said I had to hirer certai developers... through Upwork, but the people who are actually working on the app are not the same people. I don't understand why. I'm new to Upwork, but it seems odd. What's the standard practice for working with an agency?
re: "What's the standard practice for working with an agency?"
An agency can do whatever it wants with regards to personnel. What you just described in your post IS the standard practice for many agencies. Agency skeptics often refer to this as "bait and switch."
There ARE GOOD AGENCIES AND GOOD PEOPLE who work for or hire agencies. But there are also many clients who will never again hire agencies after the experiences they have had.