Application Insights
Application Insights allows you to View the Exceptions, Performance aspects of the application. You have to Enable this feature using App Service > Application Insights > Enable.
Then whatever exceptions occurring can be tracked there.
Diagnostic Logging
Azure allows storing diagnostics logging for advanced troubleshooting. The logs are saved to following 2 storage mediums.
- File System
- Blob Storage
Activity Log
Go to App Service > Activity Log. Here you can see the Subscription Level (Billing) information like Publishing, Stopping & Restart service etc.
Diagnose and solve problems
Here you can see the app performance, configuration, certificates related issues. 
App Service logs
For application level troubleshooting we have to use the App Service logs. Set the Application Logging to On with Level to Verbose. You can also make Detailed error messages to On.
Log Streaming
After enabling Application Logging & Detailed error messages you can view the App Service > Log Streaming window. This will give live logging information.
Try accessing the URL again and you should get the Logging text in the above window. Copy the text and Save as HTML file to view the Sub Level error code & Verbose information.
Advanced Tools (Kudu Console)
Open Advanced Tool (Kudu Console) > Debug > CMD
KUDU Console will give multiple log information like Application, IIS, Data, Detailed errors etc.
Console View
Go to Console blade. Run MS-DOS commands to explore the files & file contents
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