This is a good question! You will get some good answers here but my take is that neurotransmission is the domain of other fields, such as neuroscience. That’s in part perhaps because there’s still so much to know!! But also perhaps because it’s so “micro” (in a literal—not value—sense) that no single transmitter can explain for human behaviour.

Whereas, neuropsychology can be thought of as being related to overall structure & function of the brain itself. So for example, we know things about the role of the the cortex vs subcortical structures, brain lateralization, or what different lobes tend to orchestrate, & so on. Neuropsychology looks at performance/behaviour against the backdrop of what is known & normal as related to those known functions.