To function as an organism, you need to consume energy. And this normally means you also produce and excrete waste. Biologically, the cleanest you may be able to do this is to function like a plant, photosynthesise, and absorb the rarer, non-airborn nutrients through some kind of semi-passive system like roots.
This is not enough of an energy source for a humanoid like animal. A trend you'll notice in natures is that anything that gets its energy from photosynthesis, is either very, very small, or it doesn't move.
So you'd need something that provides higher amounts of energy to allow such movement.
These 'people' would also have to be very, very different to humans, as the biological functions that allow our bodies to exist and move are founded on the premise of consumption and waste. So there would be many, complicated biological systems that would need to be replaced with 'renewable' alternatives. i.e. alternatives that transition between two states using/releasing energy alone, conserving all of the involved materials for re-use. The only consumption/waste that would still be allowed, would be those involved in breathing.
Via a battery
The easiest way to achieve this, is something like a cyborg. If your 'people' have a battery, they could afford human-like acts and movements using the battery charge. However even if you had almost perfect solar-panels for skin, you wouldn't be able to capture enough energy for constant movement.
I don't know the exact numbers for this.
But I'd guess your people would have to spend days in a dormant state collecting solar energy before they could spend a single day of full activity.
So you could have other ways of providing power to this battery. For example if you had some kind of energy network your people regularly connected to. Either with a physical connection, or maybe with wireless charging to a possibly hidden infrastructure for a lost-technology setting (Why does everyone who leaves the city/major roads for too long get more and more tired until they collapse and die!?)
Or, similarly to the Silkworm Moth in Guest's answer, your cyborgs are born with a battery. To last a life span, probably a radiation based one... Which makes conception an interesting topic you may or may not want to answer.
This is probably not actually plausible, but you may be able to make a convincing narrative with it. If your humanoids live on a planet with a large enough amount of airborn 'exotic' materials that they could get all of the required materials to construct a biological organism from just breathing it in. This would require a planet in a probably similar state to Venus.
Any biological organism being able to endure an atmosphere like Venus for longer than a few minutes would be an incredibly feat. Nevermind actually harvest it for the nutrients required to continue living there.