How to make round popcorn?
Round popcorn is just made from a different variety of popcorn, called mushroom popcorn. The "normal" kind is called butterfly popcorn, or sometimes snowflake popcorn.

The two are mentioned in the wikipedia article (last paragraph of the linked section), along with a photo. It looks like it's pretty easy to buy online, if you prefer it.

Mushroom popcorn kernels are a little sturdier and easier to coat, so it's well-suited for prepackaged popcorn and especially things like caramel corn, but at least to me that sturdiness is also a slight toughness, so I prefer the butterfly-style popcorn most of us are used to.