generally talking, in analog realm, 3.
- soft clipping (diodes on the feedback loop)
- hard clipping (diods to Ref voltage)
- overloading a device (BJT, FET, MOSFET, OpAmp...)
the overdrive, distortion e fuzz pedals aways orbit around these three methods, there are not so many topologies used but the combination with the eq tailoring (pre distortion, post distortion ecc) and the components values used create millions of varieties, in some case significantly different with just the twist of a single value