Depending on the age of your Subaru, no Lightning cord is necessary for using the stereo's Bluetooth button. Make sure your phone is appropriately connected via Bluetooth (you can see your phone's contacts on the stereo's phone app). Start playing a music streaming app (iHeart Radio, Spotify, Deezer, Pandora, YouTube Music, Apple Music, etc.). Turn on the stereo and wait for it to boot up. Select Bluetooth as the audio source.
If your car stereo has Apple CarPlay, a sync cord is required. I think that Apple CarPlay came with the 2017s to 2022s. Wireless Apple Car play might have come out with the 2023s.
If your Subaru doesn’t have Apple CarPlay/the phone’s gps map isn’t broadcast (maybe 2017, definitely 2016 and older models) to the stereo screen and the stereo has an Aha app, Pandora app and a general Bluetooth input, you’ve got to connect the phone to the stereo with a lightning cord and plug it in the armrest usb. All bets are off for folks with USB-C iPhones (iPhone 15 and newer) because they'd have to get a USB to USCB-C sync cord and Apple might not allow those to transfer data. Maybe someone with a middle aged Subaru but a new iPhone will chime in.
