Main Question or Discussion Point
how transmission carry both power frequency(50Hz) and communication frequency (MHz) at a time
yup exactly which is the basis for the cursed BPL system, Broadband over Power LineA transmission line, as a linear device, can carry many frequencies (like your cable television) without them interfering. You can then tap in and selectively filter for the signals you want. No reason they can't go down to 60 Hz, or even DC.
A MHz signal source/receiver can be capacitively coupled to a power line and see very little of the 60 Hz power.