Due to federalism - each state is different. Some have open primaries and some don't. Iowa doesn't even have a primary. They have a caucus. It's not open - each party has its own thing. It's... Haha - it's too hard for me to explain.

Explainer: How the Iowa caucuses work - YouTube

What's not explained is there's a lot of yelling in the gym to try to get people to go from one candidate's "team" to another. It's a pretty silly thing to do in the era of the internet for a nation of 340 million people. Also - Iowa and New Hampshire have very small populations and they are far from being demographically representative. They are very white states.