How to get HQ zoomed in photos on pictures you already have saved?

I have a saved photo and I want to zoom in on a part and make it in HQ or nice enough to see well btw it's an iphone picture how can I do this because regular zooming makes it bad quality and you can't make it out

"regular zooming" is showing you all the detail available. You are trying to do the same kind of thing that people try when they take a snapshot and want to blow it up to wall size.

You can't.

If the image is low resolution then the detail just isn't there.