The thread-per-core manifesto has a goal of not sharing data between cores, and thus the communication inside the process becomes message passing, handing off ownership of a chunk of data to the recipient core. This lack of sharing is what enables the performance (no locks etc needed, outside of the message passing).

This is a good watch (first half is pure background, second half talks about the motivation): https://www.youtube.com/watch?v=PbgTyCSDPrs