Airplay has it’s own requirements, Roon core will convert to the highest possible format that airplay allows and send direct to the airplay device. The particular remote you use has nothing to do with it.
In Roon the audio transport is from source, via Core, to audio endpoint. Thus in your example the Core reads the FLAC file, decodes the audio to PCM, then forwards the PCM to the audio endpoint.
If the DAC does not support the source sample rate (or format) then the Roon Core will also downsample (or transcode DSD to DoP / PCM) as required for compatibility reasons.
When playing click on the Signal Path icon, which pops up the signal path window where the various transport stages can be seen in detail.