Free, Premium and Freemium

[Nomad]

The word free is self explanatory, you offer product or service for free but you use different way to make money, for instance, you can offer your mobile app for free and make money from ads.
Premium means you sell product and service for money. If your product is useful, people will be interested in paying for it.
One of the best methods is to make money is by offering it as a freemium. You offer the basic version for free and charge fees for additional services. People like to use free products but if they like the free version, they are also willing to pay a price.

 

[Grant]

Something free will usually come with no charges, that's why it's free. Freemium comes in when a game or app has paid extras like dlc or micro transactions or a paid add on. A free app/game/service becomes freemium when they add additional charges to the mix.

Premium is exactly what you said. It's a service/app/game that you pay upfront for and it too may have paid extras and subscriptions. You stand to make more money from selling your product, but if you can become a Fortnite, you stand to make more money going freemium or free to play in terms of video games.