Ipamorelin vs Sermorelin — different receptors, and that's the bit people skip
These get treated as two flavours of the same thing. They act on different receptors.
Sermorelin is a GHRH analogue — GHRH(1-29). It works on the GHRH receptor, the same pathway as endogenous GHRH.
Ipamorelin is a ghrelin receptor (GHS-R) agonist. Different receptor, different pathway, and it's usually described as selective — meaning comparatively little effect on cortisol and prolactin, which is the main thing distinguishing it from older secretagogues in its class.
Because they hit different receptors, they're often discussed together rather than as alternatives, on the logic that two pathways converging on the same output might do more than either alone. That logic is reasonable but again it's inference.
Tesamorelin is worth mentioning here because it's the one with an actual approved indication — HIV-associated lipodystrophy — which makes it the only compound in this cluster with a proper regulatory evidence package behind it.
Evidence tiers, roughly: tesamorelin approved for a specific indication; sermorelin has real historical clinical use; ipamorelin has development-stage data but no approval.