Question about FLAC and transcoding

iPhones, as I understand it, cannot play FLAC files natively. Instead, they transcode them to whatever format is acceptable to the phone.

So here's my question: if I stream a FLAC file from Plex to my iPhone, but connect the phone with a lightning cable to my head unit (which can play FLAC), will the file still be transcoded, or will the unit receive the pure FLAC file?

Just wondering if I can get away with keeping all my FLAC files intact and not have to worry about converting to ALAC (the Apple equivalent).