why vehicles use 12v electric systems? Why a seemingly strange arbitrary figure like 12? Why not 10?
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Lead acid cells have a nominal voltage of 2.1V but will often be considered as 2V. The result is battery voltage will be a multiple of 2.
The early cars had battery voltage of 6V ie 3 cells. 5V would not be possible, it is not a multiple of 2, the closes alternatives are 4V and 8V It was enough to start the engine of that era and power lamps etc. When cars got engines with higher compression ratios the starter engine needs to work harder. If you increase the voltage you do not need as high current and thick wires so the battery voltage was increased in 1950.
If you have 6V batteries and what to increase the voltage the simplest way is to initially have two 6-volt batteries. Then you can connect any part design for 6V in the middle and new stuff including started motor on the edges for a 12V system. Over time when everything runs on 12 V you can use a single 12V battery.
Trucks today often have 24V because you need to start larger engines. They are often two 12V batteries. One advantage is the weight, a single 24V battery would be twice the size and weight so quite impractical to handle.
So 10 V would not be possible if the system was created by using two identical lower voltage lead-acid batteries.
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