Aqua Regia is understood in modern chemistry as a mixture, not a single compound, of nitric acid and hydrochloric acid, typically in a ratio near one part nitric to three or four parts hydrochloric, and its unique ability to dissolve gold and platinum is explained by the interaction of its two components: nitric acid oxidizes a small amount of gold to gold ions, and the chloride from the hydrochloric acid immediately binds those ions into a stable, soluble chloride complex, pulling the reaction forward in a way neither acid could sustain alone. This mechanism, worked out well after the alchemical period, vindicates the alchemists' basic observation that the mixture's power was more than the sum of its parts, even though they had no access to the ionic chemistry that explains why.
Because aqua regia is a mixture rather than a fixed compound, there is no single modern formula to assign it the way there is for aqua fortis or oil of vitriol, and its exact strength and proportion varied somewhat across historical recipes, a genuine, acknowledged variability rather than a gap in the historical record. Modern chemical suppliers still sell aqua regia freshly mixed to order for exactly this reason, since a bottled, long-stored aqua regia is understood to lose its dissolving power well before either of its two component acids would individually expire.