They use 320 ogg for Premium’s Very High Quality, which is awesome, arguably audiophile quality and near indiscernable compared to FLAC (lossless). Ogg is much better / more compact than mp3 so you can expect much better quality at the same bitrate. It is 96 kbps ogg for Normal Quality, and 160 ogg for High Quality, both accessible in the Free tier. I am a musician of twenty years with good ears and sound equipment, years of experience listening to FLAC and converting to mp3 vs ogg at various bitrates to test, I think they’re doing a great job on this front.

support.spotify.com/us/article/audio-quality/