So it looks like there have been previous posts somewhat related to this, so please forgive me if this is a dumb question.

I am flying to BCN in April and trying to find flights. I found a decent one BOS->BCN on AA but the return appears to be on US Airways. I found it via Google but then re-found it via AA's site. If I provide my mileage# to AA I assume there's no guarantee that my return flight will be credited huh?

I am also confused how code sharing works exactly, since the US airways flight is being displayed on AA does that automatically mean it is code shared?

For reference, here's the flight I am talking about:
https://www.google.com/flights/#sear...-PHLBOS0US1730