Serial-connected and parallel-connected batteries – what is it?

A 24V system is built up by connecting two 12V batteries in series.
The plus pole of battery 1 is connected to the negative pole on battery 2.
Over the minus pole on battery 1 and the plus pole on battery 2 you now have 24V.
If both battery is 100Ah, you now have a 24V 100Ah system.
Never combine a smaller battery with a larger battery.
You will destroy the smaller battery very soon.

A 12V multi-battery system is built up by connecting two (or more) 12V batteries in parallel.
All minus poles are connected and all plus poles are connected.
If each battery is 100Ah, you now have a system of 12V 200Ah (two batteries) or 300Ah (three batteries).