Do you need supplements, or is it mostly marketing?

Mostly marketing, with a few exceptions. Whey or plant protein powder is a convenience, not a requirement — useful if you struggle to hit protein from food, otherwise unnecessary. Creatine monohydrate is the one supplement with strong, repeated evidence for strength and it is cheap. Vitamin D and B12 are worth testing for in India because deficiency is genuinely common, particularly in vegetarians, but supplement based on a test rather than a guess. Everything else — fat burners, BCAAs, testosterone boosters, mass gainers — ranges from unnecessary to actively wasteful.