It's built on RSpotify, which describes itself as:
> RSpotify is a wrapper for the Spotify Web API, inspired by spotipy. It includes support for all the authorization flows, and helper functions for all endpoints.
According to spotifyd/Cargo.toml is uses both librespot and rspotify.
Librespot is the a level playback client that reverse engineered what the native Spotify desktop apps do to get, decode and play a music stream as a connected device.
RSpotify is a high level client that uses the Spotify Web API which can control what is playing on any connected device.
Neither of which should be confused with the Spotify Web Playback SDK which turns your browser into a connected device provided it supports the right DRM bits.
It's built on RSpotify, which describes itself as:
> RSpotify is a wrapper for the Spotify Web API, inspired by spotipy. It includes support for all the authorization flows, and helper functions for all endpoints.