YouTube SEO in 2026 means optimizing four layers, not just metadata. Layer 1: use keyword-informed titles and 200+ word descriptions. Layer 2: design thumbnails for mobile (high contrast, one focal point) to maximize click-through rate. Layer 3: target 40-50% average view duration on long-form content to trigger algorithmic distribution. Layer 4: structure videos around specific questions so AI search engines (Google AI Overviews, ChatGPT, Perplexity) can cite them. YouTube holds a 29.5% citation share in AI Overviews, the #1 cited domain. Optimize all four layers to build compounding discovery.