Singnals, filters and power output
"A square signal goes through a low pass filter and in the output of a circuit we get a sine. What happens there? Give a scheme?"

Who told you this? It`s not true. A lowpass filter will attenuate the harmonics of the fundamental frequency which are beyond the path region of the lowpass. The amount of damping depends on the filter order. As a result, we get a form which is something between sinus, triangle, and "unknown". In theory, we would get a sinus for an ideal "brickwall" filter only which really removes all the harmonics which, however, is impossible to realize.