iOS 13 iPhone operating system version isn’t compatible with all iPhone models. Here are the COMPATIBLE models:
iPhone 11 Pro
iPhone 11 Pro Max
iPhone XS Max
iPhone 8 Plus
iPhone 7 Plus
iPhone 6s Plus
The listed compatible models are those released during 2015 or later. iOS 13 became publicly available at the iPhone 11 / 11 Pro launch, in September 2019.
So, iPhones which ARE NOT COMPATIBLE with iOS 13 are those released before 2015, such as the iPhone 6, 6 Plus, 5S, 5C, 5 and of course models that are even older than them.