I don't understand what is kali nethunter. Is it like a virtual box but for android?

Really thanks a lot for all the answers!

Rootless Nethunter on Android is a bit like running a VM version of Kali on your PC so like a VM it needs an external WiFi adapter if you want to be able to scan wireless networks.

There are several compiled ARM Nethunter versions and these can be installed 'bare metal' on the phones, if you've evr flashed custom ROM's then this is straightforward enough to set up and allows you to use the phones hardware, including the Wireless adapter it has built in.

For 90% of what most users need, rootless is easier and more straight forward to set up.