Salmon nigiri sushi feels like melting in the mouth. How to achieve that?
A technique, sushi chefs use is called dry curing. It means covering the fish in sugar and salt (3:1 ratio), I’ve seen recipes calling for 20 minutes to 1.5 hours. This will draw moistrure out, concentrating the taste, and will definitely have an effecf on the texture.

Another one is, freezing for bacterial reduction, try looking for fish that has been frozen at really low temperatures. When frozen, water in the muscle tissue will freeze and expand, damaging the muscle fibers, which will also contribute to the texture you’re looking for. You can try replicating this at home, except for the food-safety part, by freezing the fish for a long time, some recipes call 3-4 days.