Hello Pilou,

first: Points in the nodeeditor are also Frames. A Frame has an origin that is also a Point. In Your example X-Axis is the x-Axis. Now define a line in the shown z-Axis as your y-Axis Input. The resulting z-Axis is like your y-Axis - only in the wrong direction - but that doesn't matters I think.

That's Basic vector math. The z-vector is the cross product of the x and y vector.

Have a nice day
Karsten