India does not have a single national law governing online betting and casino gaming. Gambling is a state subject under the Indian Constitution, so each state sets its own rules — some permit and license certain forms of real-money gaming, while others restrict or prohibit it outright. There is no one licensing regime that covers the whole country the way there is in, say, the UK or Germany.
Because of that, most operators serving Indian players hold licenses issued outside India (commonly Curaçao, Malta, or similar offshore jurisdictions) rather than an Indian gambling license — because no single Indian license of that kind currently exists for most of the country. That doesn't make an operator illegal everywhere in India, but it does mean the legal picture varies by which state you're in, and you're responsible for checking the rules that actually apply where you live before you register anywhere.
Winnings from gambling, betting, and online games are taxable in India — tax is deducted at source under Sections 194B and 194BA of the Income Tax Act, regardless of which state you're in or which operator you use.
Advertising real-money gaming and betting is restricted or banned in several states, and is subject to central government advisories to media platforms and intermediaries — which is part of why you may notice inconsistent marketing rules between operators.