If it healed or helped joints — and given the ageing population in this country, that'd be a big help — the NHS would all over collagen supplement studies and looking to prescribe them, or at the least 'advise' taking them.
But they don't.
On joints, the evidence is described as "painfully weak", and most collagen supplement companies either fund their own non-peer-reviewed studies which, amazingly, find results they want
, or they selectively pick stats from generic studies, losing the context of who it helped, how, what dosage, under what conditions, etc.
When it comes to collagen supplements for beauty? Well that's worse. "No human studies have clearly proven that collagen you take orally will end up in your skin, hair, or nails". Perception studies differ but a) they're subjective and b) not controlled studies, so while it may be that '68% of people felt their skin looked plumper' that could be a variable, e.g., hydration: you're told to drink a full glass of water with each pill, the skin thrives on moisture; etc.
Sorry, bit of a long one - basically: magic cure-alls aren't found on selly telly, and when they do exist, they're not sold at clearance/giveaway/unbelievable prices by the likes of Sally Jacks.