Yamaha’s engineers did a whole lot with very little. I can only speak to the OPL2 as used in the original AdLib PC card (and to a much smaller extent the OPL3 in Sound Blasters) but IIRC all the sounds came from a lookup table of 10-bit integers representing one quarter of a sine period. Through bit shifts and addition/subtraction, out came a stream of 13-bit floating-point-ish data that the DAC converted into countless game soundtracks and sound effects.
Part of the character of the sound, aside from the whole FM-with-feedback deal, was how abrupt and coarse the envelope steps were. There was no averaging or fine interpolation; just the brutality of fast integer math.
Here's some more technical information about the OPL2, including some images of the decapped chip: https://yehar.com/blog/?p=665
The 4-operator FM synths that Yamaha produced from the DX100 (OPP/YM2164) onwards get that characteristic 'lo-fi' sound from the length of the log-sin table. The 4-operator chips used 256 samples for the quarter log-sin wave, whereas the DX7 had 1024 samples. As you increase the length of the sine table, the timbre gets much 'smoother'.
For the longest time (over 30 years) I've thought that things like the OPL3 were analog. I had no idea that those synth things were happening in numberland.
Part of the character of the sound, aside from the whole FM-with-feedback deal, was how abrupt and coarse the envelope steps were. There was no averaging or fine interpolation; just the brutality of fast integer math.