GA4 fundamentally changed how analytics works. Instead of session-based tracking like Universal Analytics, everything is an event. Page views, clicks, scrolls, video plays—all events with parameters. You install one gtag.js snippet or Google Tag Manager container, and it fires events to your GA4 property ID. Those events hit Google's servers, get processed, and appear in your reports typically within 24-48 hours for standard reports, near real-time for the real-time view. The core mechanism relies on first-party cookies (like _ga) to identify returning users, but GA4 also uses Google Signals data from signed-in Google users to model behaviour when cookies fail. This matters in 2024 because Safari's ITP and Firefox's tracking protection block third-party cookies by default, and iOS 14.5+ forces app tracking consent. GA4's machine learning attempts to estimate the missing traffic, though accuracy varies—we typically see 15-30% of mobile Safari traffic underreported even with modeling enabled. Data flows from your site to Google's collection servers, gets deduplicated by client ID and user ID (if you set one), then populates dimensions and metrics. Custom events require parameter mapping in the interface or via gtag config. Unlike Universal Analytics, GA4 has no views or filters—you control data at the property level or use comparisons in reports. Cross-domain tracking requires manual configuration through gtag's linker parameter or GTM's cross-domain settings. For our portfolio sites, we configure internal traffic exclusion by IP range, enable enhanced measurement for scroll and outbound clicks, and set conversion events manually since GA4's auto-suggestions often mark junk events. The reporting interface is deliberately limited. You get exploration reports for deep dives, but standard reports show aggregated data. For clients who need granular session replay or heatmaps, we run GA4 alongside Clarity or Hotjar. GA4 excels at user lifecycle and attribution modeling, but weak at content performance compared to Universal Analytics. Expect a learning curve—most agencies still reference Universal reports because GA4's UI remains counterintuitive three years post-launch.