Yes, the pre-injections and post-injections add more fuel to the main injection.
The value you see in the app is the quantity added for every combustion cycle.
In the normal condition, these additional injections are used to reduce emissions and reduce engine noise.
During the regeneration process, the ECU uses the post-injections to inject further fuel and increase the output gas temperature, this gas passes through the DPF burning the soot. Moreover a part of unburned fuel reaches the DPF in which it burns helping the temperature increasing inside it.
The injectors are on the top of the engine, yes, one per cylinder.