If you hook them up to an oscilloscope and look at the waveforms there is not much to all of it.
All distortion is just how badly messed up you want the sound. You can build the circuits. I've made several in the past. Germanium diodes create more tube like gradual clipping. LEDs and silicon diodes are fairly similar. The rest is pretty simple. A jfet buffer on the initial stage will help pick up more initial tone. The rest is just a couple of op amps for buffering and frequency filtering.
The easiest way to mess with the stuff is actually to get an old home theater powered sub and reverse engineer the active filter. Adding effects in place of the frequency filter is a great way to learn. Any sub is basically a guitar amp. There is no real difference in practice. In fact, the amplifier circuit and power is usually better than entry level guitar junk. Like I have a fender amp someone tossed that has a single TDA2030 chip on a 12 inch driver. That is a joke $0.80 chip. I have a cheesy junk RCA 5.1 channel computer surround sound system someone junked that is made of 6 of these TDA2030 chips, and that was junky as fuck. I've seen crap home theater sub amps with much much higher end chips.