Why are both bodies always electrified during friction? What about their charges?

One body loses electrons, another gets. When two bodies rub against each other, some of the electrons from one body are transferred to another body. A negative charge is formed on a body with an excess of electrons, a positive charge is formed on the body that donated electrons.

If more detailed:

During friction, both bodies are electrified, since a certain number of electrons from one body will pass to another, as a result, the body to which the electrons have passed will receive a negative charge (since the electron charge is negative), and the body from which the electrons have passed – positive. Brushing the hair with a plastic comb: the hair “stands on end”, and the comb itself, if shaken off in the dark, you can see sparks.