On Joined Trill memory, it could be thought of as a similar concept to “Me” in Doctor Who. The humanoid brain can’t hold multiple lifetimes of memory, but in the case of a Trill, the memory is stored in a symbiont and accessed through a complex psychological connection.

One could also liken it to sense memory, reductively, like Jason Bourne- with an amnesia that allows access to memories or experiences when exposed to emotional or physical stimuli. Take Jadzia, who before being joined had a background that was almost entirely based in scientific academia. However, when on a starship, she is almost always the pilot. Why? Put her in front of the helm and she can access the skills and experience of Torias Dax.