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The EU+UK e-privacy "cookie" rule applies to apps in the same way as anything else that's sending/receiving data over a public network (e.g. the Internet): all storage of information to, or reading of information from, the end-user device requires their free, informed and specific consent, unless it's a technical necessity for the service they requested, or certain limited (technical) purposes like load balancing. How strictly this is enforced by regulators has waxed and waned over time and from one country to another. Civil litigants, however, have had pretty good results in the courts (or just threatening litigation) - e.g. the Lloyd and Vidal-Hall cases against Google in the UK

I'm not familiar with the Instagram signup flow but it may very well be that the user did opt-in at one point. The opt-in would of course only be valid if there's also a clear "reject" option.

GDPR enforcement is significantly lacking. There's basically zero ways this is compliant unless it's opt-in (but who in their right mind would opt-in?) but the regulators aren't interested in standing up to these nasty companies.