Commercial transport passenger planes are hit by lightning an average of one or two times a year. They are designed and built to have conducting paths through the plane to take the lightning strike and conduct the currents.
Actually, aircraft often initiate the strike because their presence enhances the ambient electric fields typical for thunderstorms and facilitates electrical breakdown through air.
When it is suspected that a plane was hit by lightning, there is a mandatory inspection for damage, which can delay flights and be quite expensive. For that reason, as well as for turbulence, they avoid thunderstorms as much as possible. However, many planes are not required to be designed for protection from lightning. These include small private and experimental aircraft. There has not been a lightning-caused commercial transport airplane crash in many decades, but that's not true of the other groups of aircraft.
