Lithium battery 3.7 volts is a single lithium battery. 7.4 is the result of two lithium batteries. And 11.1 volts.
3.7 volts, the 5-volt power supply needs to be boosted. This is the mode of use of the power bank.
If 7.4 volts is supplied to 5 volts, a step-down module is required. This case is rarely used.