When the Instant Pot reaches pressure, the contents inside are actually under pressure. Think of reaching pressure as a big plunger coming down from the lid and compressing the food. When the contents are under pressure, they heat up but they have no room in which to boil. When something is under pressure, it reduces the time needed to cook something by a lot. Unless you are cooking a whole chicken or roast, it’s very rare that you need to set your time to more than 10 minutes under High Pressure.
The reason why the contents boil when you do a quick release is because the steam is displacing so quickly, the contents inside are at boiling temperature finally have room to boil as the “plunger” moves up. Contents can’t boil under pressure, even though they reach boiling temperature, because they have no room in which to boil.