What technology does Spotify use?

Yes, but it is built around a C++ core , which is also shared by the iOS and Android clients. You can say that it is built with JavaScript to the same extent that the iOS client is made in Objective-C and the Android client is made in Java.28 Eyl 2011

Is the mobile app of Spotify also based on electron? No, electron is only for Desktop .

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.