https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/typography/ Yeah, you are right, thank you. But how to specify this numbers? Do we have a rule or it's only eye measured?

This is their own font, why make it unreadable in small sizes?

You need it, if you design for same displays. In other ways, buy cheap monitor, which your users have.

Yes, typicaly 960gs, but sometimes photoshop built in tool.