> gets broken down via photolysis into hydrocarbons somehow

See Figure 2 [1]. Protons, electrons and water ions from space dissociate, in the presence of sunlight, nitrogen and methane. Those combine into intermediate-mass hydrocarbons that produce complex organics. The part we don't understand is how those complex organics, e.g. benzene and naphthalene, turn into large organic particles.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC10961852/#fig2