What is configured in Web Map and what in Studio Designer?

I just took the Studio Web course, and I'm left with lots of basic questions…

Is there something that tells what I need to configure in the Web Map (fields, symbology) and what do I need to do in Designer?  Seems like maybe some can be done in both…  

Should my Web Map be done in AGOL's coordinate system?  What about the layers in the Web Map?  Is there any disadvantage to using services to bring layers into the Web Map?