If iPhone is purchased from US, warranty will be covered in India

No. Warranty is only valid in the country of purchase.

You will need to physically return the iPhone to the US for warranty service if you need it. Apple does not take international shipments for service.

Macs yes.

iPhones being cellular and having specific hardware for the cellular network in specific regions is not covered internationally.

That is incorrect, the warranty is only valid in the country of purchase.