For those power banks with a power button, you must press the power button first to turn on the power bank. There are a few AUKEY power banks that do not have a power button that will begin charging as soon as it is plugged into the device.
There are two ways total amperage is split between multiple ports. In some power banks, there is a total output amperage for the device that is split between the two ports almost evenly. In others, there is a designated maximum amperage per port. This would be clearly stated in the product specifications. Please see the product page for more specific information for each power bank.
Capacity is measured in milliamperes (mAh) on power banks. As a general rule, the more mAh in the power bank, the more times you will be able to charge your device before having to charge the power bank again.