Your wrong there, you can definitley plug an iphone in and it will work just like an ipod, i know because thats what ive been doing for the past few years
BUT...
The newer the iphone/ios, the glitchier the experience. Eg: Back when i had an iphone 6s plugged in via usb cable, the music worked 100% fine, when i moved to an iphone xr, it started randomly disconnecting, i had to restart the iphone to get it to work correct.
But the funny thing is, my mrs has the EXACT same iphone as me, with the exact same ios but my car doesnt recognise her phone at all.
I can also plug a usb stick in, but it seems to have a limit in regards to how much music it can actually play. Eg if i put a few gb worth and say 600 songs, it will mostly play the same songs over and over (on random), so out of that 600, its only actually recognising 200