If you can get your hands on a PDF of the 2nd Edition AD&D book "The Illithiad" it could be a valuable source for deeper info regarding illithids, the Illthid Empire (aka the Grand Design), possible origins and also arms & equipment.
It's probably no longer 100% compatible with 5e D&D canon, e.g. Illthids originally had to mature for decades before becoming the Mind Flayer from the Monstrous Manual while the Absolute ones just explode out of living people with full powers and capabilities. Since I've never touched anything past 3e, I can't tell if that's just the way Illithids breed these days. But while it's not explicitly stated in the BG3 game itself, In my head canon they were a special breed created through manipulated tadpoles.
