Bunch of options...probably the easiest and lowest tech with worst sound quality would be to use an FM transmitter. Plug in to the CD player and tune your radio to the transmitters frequency and you’re done.
You could get an iPod and plug in via USB or Bluetooth.
Ditto for an Android phone or iPhone.
You could also store music on a USB memory stick.
You could pay for a service like Spotify or Apple Music where you can create your own play lists and stream via Android or iPhone.
Most of the options above would require you to download music to a device, be it Android, iPhone, or memory stick. That means you need to either rip a CD (convert the CD to individual song MP3 files), or buy digital copies of your music.
The subscription option gives you access to massive quantities of songs, but comes with a monthly cost.
The first option would probably cost less than $20 but results can be iffy depending on how crowded the airwaves are in your area. To avoid that problem when I lived in Chicago I used to have an FM transmitter for SiriusXM radios that weren’t integrated into the car. But those wired between the factory antenna and the radio and ensured a solid signal. You could still do that but it would mean you have to partially disassemble the dash.