Box 1: Health check -
Health check increases your application's availability by re-routing requests away from unhealthy instances, and replacing instances if they remain unhealthy. Your
App Service plan should be scaled to two or more instances to fully utilize Health check.
Box 2: Diagnostic setting -
Azure provides built-in diagnostics to assist with debugging an App Service app.
With the new Azure Monitor integration, you can create Diagnostic Settings to send logs to Storage Accounts, Event Hubs and Log Analytics.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/monitor-instances-health-check
https://docs.microsoft.com/en-us/azure/app-service/troubleshoot-diagnostic-logs