Rather than an aesthetic genre like "Western" or "Horror", or a mechanical genre like "First Person Shooter" or "Real Time Strategy", "Immersive Sim" is a systemic genre like "Simulator" or "RPG".
It's not defined by a singular mechanic, but a broad collection of mechanics that create a unified theme. Typically immersive sims are first person and have heavy emphasis on large sandbox of potential interactions with the world, simulating the sorts of off-mechanic nonsense you can do in pen and paper. While most games might only let you shoot or open a box to find loot, the gameplay of an ideal immersive sim comes in letting you use that box as a weapon you can throw, or as something you can stack to bypass typical level geometry.