Compose: Some questions on running Hello Docker Compose.

Still trying to understand this:

Q) The Node app exposes port 3000, but here we map 50002 to 50002, why and what if multiple ports are exposed by service?
nodeapp:
build: ./node
ports:
- "50002:50002"

Q) a single -app-port, what if the container exposes multiple ports, 80, 443?
nodeapp-dapr:
image: "daprio/daprd:edge"
command: ["./daprd",
"-app-id", "nodeapp",
"-app-port", "3000",

Q)
Do I need to stop the default containers that Dapr Init creates ?

Q)
Is the placement service only necessary when using actors ?

Q)
Will these services display with dashboard and zipking ?
If so how do I access them ? the default urls are not displaying the services.

thx... :-)