Are all the distros having the same GNU/Linux kernel

Yes. Different distros have different versions, patches and so on, but the underlying kernel is the same.

if I replace all the Arch userland files into Debian’s, the system will become Debian?

If by "userland" you mean files which your normal non-root user can touch, then no. There's differences on how distributions build directory trees, file locations, binaries, versions and so on. You can of course replace all the files on the system and change distribution that way, a convenient way to do that is to use distros installer but technically speaking you can also replace them manually by hand (which I don't recommend).