I'm glad you find it interesting. I developed the theory at university, studying how ASK and FSK modems work. To build this, you’ll need to understand the Shannon-Hartley theorem, band-pass filtering, Fourier transforms, and convolution.

For the practical 'how-to,' I recommend studying GNU Radio and SDR++; they show how to process IQ data or raw audio streams directly, and for sure there are other libraries. On the 'ancestor' side, look at the AX.25 Packet Radio protocol and AFSK (Audio Frequency Shift Keying). These are the same 'softmodem' principles used in FidoNet nodes decades ago.

GSM Arena can help you find phones with integrated FM receivers. You'll notice that many features are market-dependent, meaning: the receiver is often physically present but simply disabled by software.