The voltage of the power bank indicates its output voltage, where 1V and 2.1V are two different output voltage options. Generally speaking, the 2.1V charging bank is suitable for devices with higher voltage, such as some mobile phones, tablets, etc., while the 1V charging bank is suitable for some low-voltage devices, such as Bluetooth headphones, smart watches, etc. Therefore, you should choose different voltage options depending on what kind of device you want to charge.
The difference is that the output current size is not the same, 2.1A in addition to supporting mobile phone charging, more can charge tablet computers.
The specific use of which port to charge, depending on how much current the phone can receive, the easier way is to check the parameters on the original mobile phone adapter.
USB is 5v, there is no 1V or 2.1V, the two usb output is generally the output current is not the same, it is best to choose and your device charger charging current is relatively close to the outlet charging, which is conducive to protecting the battery, delay the battery life. No big problem.