Okay, I think you’re new to this topic and I’ll explain the basics in a simple manner.
Transformers are used to increase or decrease the voltage.
The socket that you use to power your home appliances will usually supply 230volt AC.
They produce 11000 volt in generating stations. This voltage is then reduced by the transformer to 230V to power your household appliances.
The transformer in every phone charger is able to convert the 230 volt power supply to 5 volts and then to DC to charge the phone.