What language or technology was used to develop the Spotify desktop application?

Here's the list of third-party components they use (on top of C++ of course):

• Boost
• Expat
• FastDelegate
• giflib
• libjpeg
• libogg
• libvorbis
• Mersenne Twister
• zlib
• NSIS (Windows only)
• Windows Template Library (Windows only)
• Growl (Max OS X only)
• MATrackingArea (Mac OS X only)

Given it's running on windows, clearly not .NET (Process explorer is telling me that), didn't follow a AIR install process, I'd say C++ using cross platform libraries.

Everything is compiled down into one executable, which indicates they had access to the source of all dependencies.

W.r.t to Techno...i think they used Hardhouse Electronica

This answer is more updated and coming from their engineering blog: https://engineering.atspotify.com/2021/04/07/building-the-future-of-our-desktop-apps/

The Spotify Desktop client is a Windows and Mac native application that uses CEF (Chromium Embedded Framework) to display a web-based user interface. That’s still true today, but for the previous version of Desktop, every “page” in the client was built as a standalone “app” to run inside its own iframe.

However, they recently had to update their architecture because they wanted to integrate their Web Player built with React and Desktop Client in a way that a single team can develop and ship features for both clients.

The final architecture looks like a layer of Platform APIs that expose the underlying Spotify ecosystem to clients, with a React-based user interface and the Platform APIs exposed via React Hooks. Thus, the new UI can run on the web, and it can run in our Desktop container, and never know, or care, if the data is coming from our C++ stack or our web infrastructure.

Architecture Diagram

• Issue trackers:

  • Sentry

• Security:

  • reCAPTCHA

• Miscellaneous:

  • Spotify Web
  • PWA
  • Amazon S3

• Tag managers:

  • Google Tag Manager

• JavaScript libraries:

  • core-js 3.24.0
  • Reactjs

• PaaS:

  • Amazon Web Services
  • Reverse proxies

• Envoy:

  • Cookie compliance

• Others:

  • OneTrust
  • A/B testing
  • Google Optimize