Most modern cars have "integrated" stereo systems that combine music players, navigation, cell phone, dash cams, and other features into one unit.
Upgrading the music player is no longer as simple (or cheap) as swapping in a new, standard size head unit.
On my 2014 Focus, the stock stereo was just awful. If you turned the volume up loud enough to hear music over the typical cabin sound, it would distort. It was literally so useless that I never listened to it because it sucked so bad.
In order to upgrade the sound, I had to first buy an adapter kit, which essentially replaced a good portion of the center dashboard above the climate controls and I had to swap a bunch of parts over in order to keep certain functions. The adapter kit itself was over $200.
All this stuff has to come out:
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The upgraded head unit was a basic Alpine music player that cost me only $100 new. It doesn't play CDs, but it does have Bluetooth, a USB Input, and an auxiliary input.
I hate subscription streaming audio, so most of the time I play prerecorded music from my portable music player through the aux in.
That setup, plus a pair of Infinity Kappa 6" door speakers in the front and a 12" Kappa subwoofer in the back seat make for a surprisingly clean sounding hifi auto sound system that gets low and loud, but is still very affordable.
While I did the installation work myself, I wouldn't recommend it to those without experience.
It will cost some money to have a shop install an upgraded stereo system into a newer car, but it's usually not wise to start hacking into a modern car's dashboard equipment on your own if you don't know exactly what you're doing.