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You are likely conflating two different things. "Spotify is 320kbps" is the overall bitrate after lossy compression. Spotify uses a sampling rate of 44.1kHz (I believe) before those samples are then lossy compressed. If you set Windows to 48kHz sampling, it will then resample the audio decoded from the 320 kbps/44.1kHz sampled content.
If you set a Windows audio device to 48K, Windows will send everything to that device in 48K (up or down converting if necessary). Except if an app supports exclusive mode (like Tidal and Qobuz), then the app can select the resolution.
You are likely conflating two different things. "Spotify is 320kbps" is the overall bitrate after lossy compression. Spotify uses a sampling rate of 44.1kHz (I believe) before those samples are then lossy compressed. If you set Windows to 48kHz sampling, it will then resample the audio decoded from the 320 kbps/44.1kHz sampled content.