In practice, a Power Bank is a battery and a bit of electronics that can act as a USB charger that is either plugged into the 230V mains or e.g. the 12V plug in a car.
The Power Bank is charged just as a normal phone, but due to the often significantly larger battery capacity, the power bank can often charge phones many times on a single charge.
Of course, the fact that batteries always weigh something is a given, and the rule of thumb is thus that
the greater capacity (mAh), the heavier the power bank, and the more charges it holds.
Weight in particular is a factor worth considering when traveling or keeping it in your pocket. Therefore, choose the power bank that best suits your needs or buy two different ones, e.g. if your normal consumption patterns are few or many charges in a day (or more devices that need charging). The capacity is measured in milliamp hours (mAh). One advantage may be that the connectors are at the end of the Power Bank, if you are keeping it in your pocket.
In addition, the models differ in both design and quality, of course.Do not forget to buy a USB charging cable for either your Apple or Android your phone/tablet.