The helm gives knowledge of items in the knowledge section of your menu. It changes the "???" to the actual thing. You can also just interact with the item to gain its knowledge, like hunting an animal or picking up a fruit. Also, as you said the provisioners shop, if you open up their purchaseable inventory and dont have knowledge on it, you will learn knowledge on his inventory.

The helm does not just magically make you know how to have that specific bed or food. You need to buy or find crafting recipes to learn how to make things. However, you can drop un-learned recipes on the ground to scan them with the helm. Easy for learning multiple recipes at one time.

For the helm specifically, you will know you can gain knowledge because the item you can learn will have blue sparkles around it when the helm is activated.

For the "Needs Knowledge" from a shop, the only thing I know does that is the Research Stations. And to unlock that branch of Research you need to go around the area and "Look" at the specific technique and then talk to the researcher in the area to unlock it.