Answer: The plant uses the food stored in the bulb to get started growing. As you can imagine, the bulb stores much more food than a tiny seed can, so the plants can grow much more quickly. To get the bulb-less scallions like you get at the supermarket, you'll need to plant onion seeds. Started in early spring, they'll grow into the green onions, and, if left alone, will begin forming a bulb in the fall.
What you have harvested are essentially baby onions--but you can use them just like scallions, bulb and all. You can pick them all now, or thin them until they are spaced about 6" apart, using the thinned ones as scallions and allowing the others to mature into bulbs for fall harvesting.
By the way, welcome to the world of gardening. Aren't plants fascinating?
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