How are cores/threads allocated?

Given that applications must be written to take advantage of multiple cores, I know that Photoshop will take advantage of extra cores but I am not sure which other applications are optimized for multiple cores/threads. Where can I get this information for specific applications? Also, when multiple applications are being run simultaneously (possibly by different users) how are cores allocated? I would assume that first each application and/or user would be allocated a core/thread if available and then any application which could take advantage of multiple cores/threads would be allocated additional cores until all resources have been allocated. Correct? If so, what happens if there are not enough cores/threads and some must be shared?

Thanks in advance for any insights.