Below in the BT plugin, you have two options for streaming with instructions. Follow either one to get streaming from your phone to either a channel in your Squeezebox Server or directly to your sound card.
Since you want the latter, make sure the player is running and you have your output device selected. Once this is case, the other players only to open the channel for BT output to go to your hardware. This is achieved with Squeezelite by the option „-C 5“ which automatically disconnects the player form the hardware after 5 seconds of inactivity. Select „sndrpihifiberry…“ for your sound card’s audio output in the Bluetooth player (called bluetooth speaker, this is indeed confusing but the name of the playersoftware). Afterwards, make sure Shairport and MPD are not running and you should be able to stream BT to your sound card (even with Squeezelite running but inactive).