⚠️ The Lumia 520 runs Windows Phone 8/8.1 , not Android. Custom ROMs here are not like Android custom ROMs (LineageOS, etc.). You cannot install Android on a Lumia 520 (except very unstable, outdated experimental ports). This guide covers what is actually possible.
The core of the "magic" lies in the registry. Custom ROMs inject a modified boot.stm and BCD (Boot Configuration Data) that lie to the Windows 10 installer. The phone reports: nokia lumia 520 custom rom work
And that’s beautiful.
Custom ROMs offer a way to revitalize older devices like the Nokia Lumia 520 by providing new features, updates, and enhancements that may not be available through official channels. They can also help to: ⚠️ The Lumia 520 runs Windows Phone 8/8