Mostly Python. Django for web app and rest API. The Falcon micro-framework for telemetry collection API.
The dashboard is a React SPA (since 2014!) that is powered entirely by our REST API so new features are always API-enabled.
Alerting rules are handled by a Python daemon that processes the event stream.
Healthchecks are dispatched by coordinators to Lambda workers in a half dozen regions around the world. We started using Lambda before the launch of great frameworks like Serverless so we had to roll our own lambda deployment tooling with Fabric and Boto.
https://ponyfoo.com/books