When batteries are used in parallel, the battery voltage will not change, but the capacity will double. Only when used in series does the voltage double. For example, the voltage of a single lithium battery is 3.7V and the capacity is 1000MAH. If two are used in parallel, the voltage is still 3.7V and the capacity becomes 2000MAH. If used in series, the voltage becomes 7.4V and the capacity is still 1000MAH.
Parallel connection is a method of connecting components. Its characteristic is that two components, devices, etc. of the same or different types are connected head-to-head and tail-to-tail. Usually used to refer to the method of connecting electronic components in a circuit, i.e. a parallel circuit.