Questions regarding PCR
Ideally amplification happens between the two primer pairs and results in one product. However, complicated secondary structures, the depletion of some of the reagents may cause pauses and fall off of the polymerase. This happens often later in the reaction and results in shorter products. This final amplification step is introduced to finish all these shorter products.
A pair means the combination of a forward and a backward primer (see the image below from here). Bothg primers are added in a concentration high enough that both are not depleted and enable long PCR runs. No primers (or other reagents) are added during a standard PCR reaction.





The temperature of the PCR reaction is changed through 1 reaction cycle: First you start with the denaturation of the DNA at 94°C, then cool down to anneal the primers (typically between 50 and 60°C, depending on the primers), followed by the amplification step at 72°C (it'S length depends on the length of the amplified DNA piece, Taq polymerase processes about 1kb/min) (see figure below from here). This is one cycle and you know where you are, because the PCR machine counts this cycles.