Kosher sushi is kosher because it uses kosher fish. There’s a lot of info online on how to prepare salmon. For safety reasons it legally has to be frozen for 72 hours (in a commercial freezer) to kill any parasites before selling it raw, so it’s not exactly fresh. I would think the sushi places buy it frozen for this reason, but I’m guessing here.
To taste good I think the trick is to soak the salmon for a short while in a vinegar mix. Look up recipes as I don’t remember the exact details.
You can also buy kosher frozen sushi grade tuna, which saves you the worry about parasites. And since it’s probably frozen when really fresh, it may actually taste better than something questionable, or supermarket salmon.