We have lots of first time gardeners as customers. Some things around gardening and growing are just confusing. We try to make it make it simple so we found a chef to drop some knowledge:
"They're all alliums, along with onions, garlic and shallots. Onions garlic and shallots, we use the bulbs. Leeks, scallions and chives we use the green/white stem. Leeks are the biggest and I think the mildest flavor. They are usually cooked.
Scallions/green onions are usually used uncooked as final flavor/garnish. When cooked they can take on a soapy flavor if sauteed much. But they're good grilled whole. I think these have the strongest flavor and to my taste have a bit of hotness to them.
Chives are a thinner plant, though there are two varieties. The slightly broad flat leafed kind are the classic baked potato garnish and have a mild onion flavor with just a bit of green earthy sharpness. This is the kind most often used. The other kind is even thinner and usually called garlic or Chinese chives. As noted they do have a garlic/onion flavor.
With chives the tip is sometimes a bit wilted so don't use that. Towards the base, they can be a bit woody and tough too. The blooms from chives are sometimes used in salads/garnishes as they are edible. I'm not fond of eating the blooms--flowery garlic is not a flavor I find pleasing.
I don't consider them interchangeable. The flavors are different and so is the texture. As noted, leeks are usually cooked, green onions may be cooked 50/50 or raw, and chives are rarely cooked or added for just the last couple of minutes as the seem to lose their flavor if cooked much."