Threshold-based alerts are also Free / Basic but with limited connectors (log, index only)
Threshold-based alerts with connectors to Email, Slack, Teams etc..etcc. require a subscription
Times Series Anomaly Detection / Alerts (Machine Learning) requires a subscription. This allows elastic to learn what is normal and detect abnormal conditions / events and hen drive alerting.
Version: If you are starting now, you should start with a newer release in the 8.x series each release adds significant capabilities, APM and alerts and anomaly detection have many historical releases, I would not go back any further than 7.17.x