FM is very simple implemented on the Digitone.
The Operator Models are Like the Oscillator from a Classic substractive Synth. They can be detuned and their Output can be Mixed, also Like two separate Oscillators. Than the Rate A and Rate B settings are Like setting the Oscillators frequency.
Additionally each Operator Part (read each “oscillator”) has its own envelope which Output can also be mixed into the Signal path.
Then you have a LPF and a HPF to further shape the Sound. Amp and Filter envelopes, two LFO per track for Modulation and three master FX Slots. Thats basically all you need to know. From there it helps to study the presets and See and learn which Operator Model helps with what Type of Sound. Thats it.
Here´s a nice explanation about FM Synthesis from Loopop: https://youtu.be/x2Ic9upij-U?t=123