Simplistically, to achieve a big flavourful IIPA you need: Big Malt + Big Hops + Big Fermentation.
It’s a bit of complicated symbiosis between these three requirements and each of these three provide multiple elements. The malt provides the complex spectrum of sugars for fermentation as well as the main structure and body of the beer. The hops contribute bitterness to balance the malt sweetness as well as resinous piney flavours and juicy fruity-citrusy aromas. The fermentation provides us with a big hit of fun juice (ethanol) and the vast array of incredibly important characterful aroma and flavour compounds.