Is it a question as to whether you can charge the iPhone and mobile battery at the same time when there is only one outlet? Unfortunately, it can not be charged in parallel, but if it is a mobile battery that supports "pass-through charging" , You can charge the mobile battery after charging the iPhone with priority.
Pass-through charging is a function that preferentially supplies power to connected devices such as smartphones without suddenly storing the power input to the mobile battery. Although not parallel, the mobile battery is charged after the connected devices are fully charged, which creates the advantage of charging multiple devices and mobile batteries even if there is only one outlet.
If this does not support pass-through charging, the mobile battery will either be fully charged but the iPhone has not finished, or the iPhone will be fully charged but the mobile battery will not. In general, the mobile battery is prioritized, so you can think of the latter condition, "the mobile battery is fully charged but the iPhone is not charged".
There are many mobile batteries that support pass-through charging, but choose a battery with a large output current (amperage). Since the recent iPhone supports 12W (18W for iPhone 11 Pro/Pro Max) input, if it is a mobile battery that can output 2.4A, charging will be completed in less than half the time compared to 1A To do. However, there are mobile batteries whose output drops when multiple devices are connected, so let's check in advance.