Timeline of a GW190403-like merger event that occurred inside one of the computed cluster models with W₀ = 9, Z = 0.005. The meanings of the symbols are the same as in Fig. 5. Here, early in the cluster evolution, the merger primary BH was born in a primordial binary and gained mass via BH-TZO accretion from its stellar companion. The merger secondary BH was born in a symbiotic binary. This is how both of the merging BHs started maximally spinning (a₁ = a₂ = 1), given the present scheme of assigning BH spins (Sect. 2). As in the example of Fig. 5, the BBH merger happens due to the dynamical pairing of the merging BHs and, thereafter, a triple interaction involving a third perturber BH. In this case, both interactions happened only a few million years before the merger (lower panel). All other GW190403-type mergers in the present models are also in-cluster dynamical mergers, involving at least one such ‘spun-up’ BH, except for one which is an ejected merger (see Fig. 3).