Good questions. If you use OnTriggerEnter or OnCollisionEnter these functions seem to work independently.
If you create your own custom function like you did, Unity has no idea when to use that information. So when you use it in the Update function it’s continually updating to see if anything has changed. If it detects a button input, then it proceeds to do whatever you had in that specific function. Let me know if you need further clarification.
