Hume famously held that all that was meaningful in the mind consisted of clear and distinct impressions and ideas. Now, notions such as causation, unity and identity are held to be a result of associative thought, in which such things as similarities between impressions and the tendency of certain impressions to follow upon others form respectively our notions of identity and causation. Now, since impressions are 'distinct and clear', in that every impression is truly unique and distinct from the other, how can such associations occur? Does not the fact of association suggest that our thoughts are produced by an active agent capable of conceiving completely distinct impressions as connected?
Our perceptual mechanisms are not passive. To a large degree, they work like sciences do, generating and confirming guesses, rather than assembling causal relations out of individual pieces: https://www.braindecoder.com/up-to-90-of-your-perception-could-be-made-up-purely-by-the-brain-1104633927.html
Association, then, is the norm, and distinctions are discovered by an active process. This goes along with William James notion that the infant mind is a "blooming, buzzing confusion", that our ideas originate con-fused (bound together, associated), not distinct, and that we grow into clear perceptions and ideas by separating, not combining. At a different extreme, it is a common aspect of Buddhist practice to assume All is One and distinctions are created by attachment.
So there are a range of mutually reinforcing psychological approaches, ranging from sheer introspection to observations of learning to neurological timing data, that credibly controvert the more physical, atomistic notion that perceived events are individual things in need of correlation, which Hume struggled with.
The idea that our actual ideas are 'pastiche' and not 'sculpture' is a side effect of our need to adapt them into stories, which have a succession of plot points and are told in assembled structures like sentences and words. We are such social beasts that the social reconstruction of our actual thoughts seems more realistic to us than their real, underlying forms.
So "There is no 'there', there." There is no need to explain how we put ideas together, however basically unrelated they may seem, only why we distinguish them.
Concerning assocation Hume writes in Book 1, Part 1, Section 4 of his A Treatise on Human Nature:
The qualities, from which this association arises, and by which the mind is after this manner conveyed from one idea to another, are three, viz. RESEMBLANCE, CONTIGUITY in time or place, and CAUSE and EFFECT.
Later he reduces the relation of cause and effect also to the first two ideas, see 1.3.2.
Hume does not pose a distinguished actor who achieves this assocation. His point is the absence of such an actor - e.g. called the soul.
The ideas themselves create theses relations. You may call this either active from the viewpoint of the ideas, or passive, because there is no distinguished actor. Which wording, active or passive, applies that's not Hume's point.
The viewpoint of neuroscience resembles very much Hume's description. If the face of a certain person is stored as a pattern of activation of a set of assemblies, then the impression of only a part of the face creates a similar activation of a subset of assemblies. This subset activates the rest of the original set by existing connections within the whole set of assemblies. That's the mechanism of assocation.