How do you make money off freemium?

Excuse me, could you elaborate on how one typically generates revenue through a freemium business model? As I understand, freemium involves offering a basic version of a product or service for free while charging for advanced features or additional functionality. But how does this translate into actual income? Do businesses rely primarily on upselling or do they employ other strategies like advertising or data monetization? I'm particularly interested in understanding the mechanisms behind successfully monetizing a freemium offering.