The TIDAL client, on both Mac and PCs, will let you choose to output directly to any audio device on the system (it’s under “Settings | Streaming | Sound | Sound Output” in the TIDAL application).
This is useful if you only want your music to go to the Dragonfly Black (or any other DAC) but not system sounds. It’s also useful when you have virtual sound-cards installed that you don’t want handling all of your system audio.
You can, of course, also do it via the System Output as @pwjazz describes above.
There’s nothing to switch. The 3.5mm output on a Mac Mini is both a TOSLINK output and a variable-level line/headphone output. You don’t need to do anything to “switch it”. Audio directed to the “Digital Out” will drive that connection in TOSLINK mode, and the “Internal Speakers” setting will drive it as headphone/line-out.
All of the streaming services will output 16/44.1 data. I expect you mean lossless 16/44.1 output - i.e. in FLAC (or ALAC) format that hasn’t been lossily compressed using MP3, AAC or OGG/Vorbis. In which case, TIDAL premium will do that (the cheaper TIDAL tier is still lossy).
If you have a local library of files, multiple devices, and want to combine that local library seamlessly with TIDAL content, give the Roon trial a go … makes for a much better overall experience AND it can talk to your Sonos devices as well as your computers and portable units.