Slightly off topic: My iMac is connected to an
Elecraft K3S, but that's a very high end transceiver, and way overkill for just receiving weak digital signals on a computer, but I do much more with the K3S than just weak signal propagation reporting.
The K3S is an analog super-hetrodyne transceiver. What that means in simple terms is the radio uses analog electronics to receive and process radio signals, and it presents itself to the computer as a generic USB audio device. The computer can both send and receive via USB audio. And the computer commands the radio via RS-232, such as Push To Talk, frequency selection, filter settings, etc. When it's up and running, all the radio controls are on the computer. The legacy interface (USB Audio and RS232) makes it easy to control and operate this radio with an old iMac.
Most modern radio receivers are Software-Defined Radio (SDR) receivers. What that means is an analogue signal is converted into digital data, and a CPU uses software to process the signal and create audio. Some SDRs have an internal CPU that does the digital processing, and appear to the computer like a traditional analogue receiver. Other SDRs rely on the computer to do the actual radio signal processing. If the SDR offloads the signal processing to the computer, then I would recommend a more recent Intel- or ARM- based mac.