@Fortune All effects that have the j prefix are taken from the juce::dsp module. In this case it's that class:
https://docs.juce.com/master/classdsp_1_1Compressor.html#a20c5600e7d04e0afcf07a74c2926cb76
The standard "compressor" is the simple chunk dynamics suite from here:
https://github.com/music-dsp-collection/chunkware-simple-dynamics