"I was just wondering if the Macbook Pro 2020 had a line-in jack? If so, what cable should I purchase so I can plug in a non-USB sound mixer in?"
No analog line in.
The only practical "ways in" will be through the USB ports, or to connect via a thunderbolt device (such as an audio interface).
There are relatively cheap devices that will convert a 2-channel (stereo) analog signal to USB.
That will probably work.
You can fight with this, but the best solution might be to get a mixer that has a USB OUTPUT.