Bangle.js 2 - Is the firmware open source?

I have recently bought the Bangle.js 2. I was searching about the source code for the firmware of the device and I couldn't really find any.

All that I managed to find is the source of the JS interpreter at https://github.com/espruino/Espruino

Can someone share the link for the firmware code? Or is it closed source?

Yes, that is the firmware you need. The building guide is https://github.com/espruino/Espruino/blo­b/master/README_Building.md building in your case with a BOARDNAME of BANGLEJS2, that will build the bootloader and espruino interpreter to install (along with the bare minimum bootstrap apps)
Once those are up and running then the functionality and apps are provided from: https://github.com/espruino/BangleApps again there is a readme there with a lot of information on how you can build your own and/or create your own forked "app store" to use.

Basically there is a mix of C++ and Javascript that makes up the working environment. The build tools will build a combined firmware "hex file" that combines the bootloader, the firmware and the basic bangle bootstrap apps.
Obviously it all depends on what you're interesting on doing or working on for which bits of the environment you actually need to build.

Thanks for the info. I'll check it out. Mostly I'm just interested in designing custom apps but I also wanted to know about what we can do at the firmware level.