Batteries are connected in series when the terminal of a battery is connected to the terminal of the opposite sign of another battery (+ with - and - with +).

In the second image, the batteries seem to be connected in parallel, but if you pay attention to the arrangement of the cables, you will see that the base of one battery (-) is connected to the head of another battery (+).

When batteries are connected in series, the system obtained has a voltage equal to the sum of the voltages of each battery, but the resistance of the batteries is also equal to the sum of the resistances of each of the batteries.
If you wish to get a voltage (tension or DOP) of 6 V you should connect four 1.5 V batteries, and if you wish to get 9 V, you should connect two 4.5 V batteries.

You should never connect used batteries in series with new batteries, or use different types of batteries. If you do, the efficiency of the batteries will decrease because the new batteries will start off having to oppose a resistance that is greater than the one caused by a new battery. Remember that batteries offer some resistance to the flow of electricity (although they also supply electrical energy at the same time), and that this resistance increases as the battery ages, causing the voltage supplied to decrease.
Change the parameters at the bottom of the attached visual to study the properties of this arrangement of batteries.