Installing linux on an android device?

[MasterP48]

Lately I've been reseaching different things to do with an old android smartphone, and i came across a couple of apps that "install linux" in your android device (eg: linux deploy), but they don't say if it's like an emulator or a virtual machine or if they effectively remove android and install anotger distro. My question is, how do they work? Is it reversible (if they don't remove android and i'm still able to use whatsapp and usual apps i will install it on my main device, that's why i ask)

Ps: do i need some kind of special requirements or something?

[Electronics Wizardy]

Android is linux, and you can't put a different distro without lots of work, and there are almost no other mobile optimized linus os's.

[comicsansms]

Android is linux, and you can't put a different distro without lots of work, and there are almost no other mobile optimized linus os's.

What about Firefox Phone OS or Ubuntu Phone OS?

[Aniallation]

I installed Ubuntu on my Nexus 7 using MultiROM and it actually worked quite well. I don't see a practical use for it, though.