iPhones and Android phones run different operating systems.
Although there are many developers who create versions of their apps for both mobile operating systems, an app that is written for one mobile operating system is not compatible with the other. So you cannot re-download paid Android apps onto your iPhone/iPad, or paid iPhone/iPad apps onto an Android device.
Within the Apple ecosystem, if you purchase an app and download it to one iPhone, you can re-download that app to another iPhone registered to the same Apple Account.
Redownload apps and games from Apple - Apple Support
With some third-party apps where you pay the software developer for a "subscription", you may find that there are both Android and iPhone versions of the app; and you can download the iPhone version, put in your "subscription" details; and use the iPhone version without having to pay a second time. E.g., I think if you subscribe to Microsoft 365 (what used to be Office), that probably would include full use of mobile apps both on Android and on iOS.