They're an alien creature that lives many lifetimes, takes over hosts (with their consent) and in return extends the host's lifetime. As a result the creature often changes gender (and species) when switching hosts. Canonically it's established that they have the host's consent but the series never really expands on that, it definitely seems murky as the creature seems to have complete control over the host's body, though it's hinted the host changes the creature's thoughts and behaviors to an extent.
That's hilarious about Dax claiming it's consensual but never explaining it. It's a "slaves developed skills which, in some instances, could be applied for their personal benefit" vibe.
It's explained as a true symbiosis that goes both ways. The symbiont's mind changes just as much, and the host has a bevy of "past lives" to draw on the memories and skills of.
It's considered a great honor to host a symbiont, to the point that they have to lie about how compatible they are because the symbionts themselves reproduce very slowly and are picky about their hosts. There's not enough for every Trill to have one.
I loved the character of Ezri. In an odd way, she's very relatable. Not the symbiont thing, but the "suddenly thrown into a situation out of her depth and trying to cope" thing. We've all been there at some point.
It's explained in the TNG episode where the Trill and the symbionts are first introduced; iirc, the Enterprise carries some candidates for a joining, Crusher (or maybe Troi??) gets romantically involved with one of the Trill (maybe the host that is about to pass?), and through that relationship we learn some about the process for choosing a new host and that it's a very coveted position. In DS9, iirc, it's part of Jadzia's backstory that she got into some of the studies and extracurriculars she did specifically because she wanted to be an especially appealing host when presented to the Dax symbiont.
You should do it! My partner and I are in the middle of watching the entire 90s run by air date (I set up a smart playlist on my plex server to make this automatic). It's a lot of fun, we just finished TNG, DS9 is getting going after laying down its foundations, and Voyager just began.
I'm not far enough to judge Voyager just yet, but I can agree that up that Mulgrew is doing a fantastic job with some very "season 1 of a Star Trek show" stories. If she keeps up what she's doing, she will definitely carry me through the show.
Unfortunately, every season of Voyager is basically a "season 1". There's some gems scattered throughout but it never really matured into its own, the way TNG and DS9 matured into their own. There were rarely any long-lasting narrative consequences resulting from individual episodes. A lot of the actors visibly stopped caring after awhile. It didn't help that the consultant hired to flesh out the character of Chakotay was a fraud.
i watched voyager first and when i moved on from it i realized just how much mulgrew carried the show. picardo too. i liked the actors more than the characters.
I feel like it's sorta the other way around- when Dax moves to Ezri she has a completely different personality. She's not a great leader, very lacking in confidence etc. I'd say the symbiont has a substantial effect on personality but otherwise acts like a store of memories for the humanoid host.