Gradle is a build system. You can build your Android application without one, but, in that case, you have to use several SDK tools by yourself. That is not simple! This is a part where you need a Gradle and Android Gradle plugin.
Gradle takes all the source files and processes them by tools we mentioned. Then, it packs everything into one compressed file with the .apk extension. APK can be uncompressed. If you rename it by changing its extension to .zip, you can extract the content.
Each build system uses its convention. The most important convention is about placing source code and assets in a proper directory with proper structure.
Gradle is a JVM-based build system, so that practically means that you can write your own ...