50 hertz sine wave in oscilloscope when I touch it

There is a small capacitance between you and the power network. Also there is another, different capacitance between scope's ground and power network. Input impedance is high enough, usually 1Mohm, so you create a kind of voltage divider between power network, oscilloscope's probe and it's ground.

This is a purely capacitive phenomenon. Calling it an "antenna" is a bit misleading and is not any better than others slogans.

You are acting as a big antenna and you are picking 50Hz from the mains.