I think for example for oscilloscope what gets signal just in one wire and how can even ac go through only one wire?
AC current does not need to have a closed conductive path (a closed circuit), because it does not transport charges systematically in a single direction along the conductor - the charges, apart from wandering randomly in the conductor, just oscillate around their average positions. This oscillation produces variable electric field that can pass small gaps between conductors, if the gap is not too wide and if there is a lot of charges involved at the end of the conductor. Then the oscillation is propagated even across such gaps.
For example, in a capacitor, which separates two conductors via nonconductive gap, the amount of charges is big due to big area of its plates, and the plates are also very close together, which makes capacitor very conductive even if it actually is place where the circuit is galvanically disconnected.
An oscilloscope exhibits what is called extremely high input impedance, which means even extremely tiny AC voltages can be detected by it. This means that even if there does not appear to be a deliberately-connected ground line between the chassis of the O-scope and the chassis of the device being tested, there is enough capacitive coupling via stray capacitances to ground that extremely tiny net currents can flow through the device-to-O-scope connection that the O-scope will indeed respond and detect the AC voltage present on the "hot" side of the O-scope input.
Note that for small capacitances like these, the resulting AC currents are far too small in most cases to be lethal to humans.