How do people develop WhatsApp mods like GBWhatsapp, etc (with extra features) when the source code of WhatsApp is not available?

Question

Many third party whatsapp mods are developed with so many cool features like automatic replies, notifications when a person of interest comes online, message timer, etc. When whatsapp hasn't released any source code and with reverse engineering becoming too tough how are these guys able to build such features?

Answer 1

They use reverse engineering, tools for reverse engineering 3rd party, closed, binary Android apps. These tools can decode resources to nearly original form and rebuild them after making some modifications. It also makes working with an app easier because of the project like file structure and automation of some repetitive tasks like building apk, etc.

• Disassembling resources to nearly original form (including resources.arsc, classes.dex, 9.png. and XMLs)
• Rebuilding decoded resources back to binary APK/JAR