How does the event-based data model work in GA4?

In GA4 everything is an event. A pageview is an event, a click is an event, a purchase is an event. Each carries parameters giving context.

There are four categories:

• Automatically collected — first_visit, session_start, user_engagement. No configuration needed.
• Enhanced measurement — toggled on in the data stream: page_view, scroll, click on outbound links, file_download, video_start, and site search. Considerable value for no implementation work.
• Recommended events — predefined names and parameters Google expects for common actions, such as purchase, add_to_cart, login, and sign_up. Use these names exactly, because reports and integrations depend on them; a custom event called purchase_complete will not populate e-commerce reports.
• Custom events — your own, for anything specific to your business.

Conversions (now called key events) are simply events you mark as important. Any event can be flagged, which is far more flexible than Universal Analytics goals.

Note: Two practical limits worth knowing. Custom parameters must be registered as custom dimensions or metrics to appear in reports — a very common reason data appears missing. And there are caps: 50 custom dimensions and 50 custom metrics per property, and 25 parameters per event, so a measurement plan matters rather than tracking everything.