In cellular communication, the coverage of one cell is very limited (several km radius in rural area and several hundreds meters radius in urban / dense area) but you can maintain a communcation(e.g, voice call) for several tens of km while you are driving. How this can happen ? It can happen because a network relay the UE(e.g, mobile phone) to next cells when the UE is getting farther away from the current cell and is getting closer to another cell. This kind of relying mechanism is called Handover and this is a crucial mechanism in cellular communication.
Overall Handover mechanism is same in 3G/4G/5G, but the detailed parameters and mechanism varies a little bit depending on Radio Technology.