The automated system was trained on hundreds of thousands of reports which made up tens of thousands of prior punishment cases from the Tribunal system, where players voted on whether each others behavior was punishable or not. The system is based on the standards the community set for itself many years ago, and is dynamic to this day - if Riot sees a specific behavior being reported a lot, they may add it to the things the IFS (automated system) looks for.
Further, the system does not monitor all games, but only monitors those games in which someone was reported by at least one person - and it only takes one report to have it review the game - it doesn't review any "harder" with more reports.