YouTube's 2026 Culture & Trends Report reveals that "mainstream" hasn't disappeared—it has fundamentally shifted from top-down (traditional media gatekeepers) to bottom-up (communities, creators, and fans driving trends). Key metrics now include visibility across platforms, comprehension across diverse audiences, and active participation (comments, shares, remixes, purchases). The report highlights niche content becoming mass content through community engagement (e.g., "The Backrooms" meme lore attracting 90M+ YouTube views, then becoming an A24 film grossing $118M). For SME/tech audiences: community-driven content strategy is now essential; brand success depends on supporting creators authentically and earning community endorsement rather than buying reach alone.