system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it if a stereo unit cannot decode a lossless audio format (e.g. flac) it can still receive lossless audio from another source (e.g. phone) and play it back at lossless quality?

My car stereo doesn't support FLAC for playing music on usb or sd card storage, but I can stream lossless music from my phone (via Bluetooth 4 or auxiliary) to it. Surely it must support high quality audio codecs if it can play them back via an alternative source?
gpt
It doesn't support the codecs, at that point it's receiving the output of the phone, which is an analog output (in the case of the headphone/output jack) or a mutually supported digital output. 

To ELI5 analogy it: humans can't digest grass, but we can digest beef. Just because we are eating a cow that ate grass doesn't mean we can suddenly digest the grass that made that beef possible. Likewise, just because a stereo system can accept an analog signal generated from a FLAC file doesn't mean it can accept the FLAC file. The phone converts it to a readable signal.