[gambiting]

I mean, both your PC and your phone will have special DRM-compliant hardware decoders available. Whether Spotify is using those or not is another question, but yes, the CPUs in phones/PCs are equipped to decrypt DRM-secured content.

(one "interesting" case around that was that some OnePlus phones couldn't play netflix in resolutions higher than 480p, because despite the Snapdragon chip supporting the DRM format required for higher resolutions, OnePlus never paid the licence to use it - so their phones couldn't decrypt the video. They did relent later and said they will enable the functionality for users who send in their phones, and it can only be flashed in person at their service centre, not via an OTA update. I always wondered how many people have done that)