are there guitars that habe the strings go fully down the body?

Like a steinberger?

There’s something called the Chapman Stick

You can get pretty close on any guitar by mounting a bigsby to it

But what (I think) you’re asking for essentially is a guitar where the actual vibrating length of the string is equal to the overall length of the instrument. This is not often done, as it limits your options significantly in terms of what standard components you can use (such as if you want to switch to a higher mass bridge).

But they exist! Look up the strandberg boden line of guitars. Headless + bottom tuning, although there is still a bit of material on either side

I know nothing about guitars, but I've seen Ichika Nito with these ibanez guitars. They don't have a head, and the strings go all the way to the other side, making the guitar more compact.

Like a banjo?

Like a cello?

It gives you less fret board space to do so unless you mean it also has a bridge in there at some point.

A lot of acoustics in the 20s and 30s have very long tail pieces and a floating bridge like violins that are kind of like this.

nee dass ist unmöglich

There's that sub-octave bass guitar that might be close? I'm not really sure

I have seen classical guitars that do, not unlike a cello, but cannot steer you at a specific brand.