Reductionism is about explanation.
Reductive explanation is an explanation of a high level feature in terms of its parts and their interactions. Reductionism is the idea that reductive explanation can or should apply to everything. Note that the existence of component parts is necessary but insufficient for reductive explanation: you can’t have a reductive explanation of something that is not made of parts, but noticing that something is made of parts does not itself give you an explanation of its behaviour
Reductionism is about explanation. Reductive explanation is an explanation of a high level feature in terms of its parts and their interactions. Reductionism is the idea that reductive explanation can or should apply to everything. Note that the existence of component parts is necessary but insufficient for reductive explanation: you can’t have a reductive explanation of something that is not made of parts, but noticing that something is made of parts does not itself give you an explanation of its behaviour