The Files App for the iPhone is provided by Apple. You can download it (again?) from the App Store. If you are not offered a download option, it is already on your phone and an "Open" button should be presented to you.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
