The Power Bank that we talked about in my earlier post usually comprises or 1 or more 18650 cells - But these are connected in parallel.
There are wild claims regarding the capacity of power banks and 18650 cells as well. So lets make it clear here - The Size of a 18650 cell is 18x65mm. In that space there is little room for wild swings in capacity - The Chemistry is nearly the same. So we have real capacities ranging from 2,000mAH to 3,100mAH.
Cell capacities are Colour coded (You can check the details in one of the links below). The Purple Colour Cell manufactured by Samsung is one very commonly available battery and has a capacity of 2,600mAH - You can take this as a benchmark for checking most of your products.
Hence if you have a Power Bank with 2600mAH it will have one cell.
For 2 Cells you can get a capacity of 5200mAH
The 10,400mAH Power bank will have 4 cells in parallel.
In Parallel - the cells will have the same voltage of 3.6 to 3.7 V - They have a full charge voltage of 4.2 to 4.3V. The Capacity will add up according to the number of cells.
Note that In series connected batteries like in your laptop the capacity will remain same but voltage will add up. Another important aspect is that if you have to use multiple cells in series, then a BMS or Battery Management System is usually used to prevent a single cell in the series from getting overcharged or undercharged. Such a system monitors each individual cell in a battery with multiple cells in series.
A note about your power bank - The Input as well as output of a power bank is 5V - This is standardized as per USB standard. The Battery Voltage is upconverted for the output using a DC-DC converter and for charging a special IC takes care of providing the correct Voltage and Current to charge the battery.
Links for further reading;
1) Battery Chemistry Explained
2) How to Identify 18650 cell capacity by Colour
3) Understanding Tesla's Lithium Ion Battery