Hey there @Kelsey
Input/Output + Dry/Wet for the FX section : you have to think audio signal and analog workflow
So like analog gears, the more you send/inject signal into the device, the more it will react, saturate, distort...etc. So the INPUT knob is there to give you control about how much of the audio source you're "injecting" into the FX algorithm and get either a more subtle or very pronounced effect at the output. Driving harder an effect with more input can lead to very different flavors.
The OUPUT knob gives you control of how much you want the effect to be heard.
Usually, you will start with the Dry/Wet knob set to a max value in order to listen what the effect is actually doing to your source signal, then adjust the Input & Output knobs to taste and dial back the Dry/Wet knob to blend with your source signal.
Velocity Knob on the Envelopes Controls : this is controlling the velocity sensitivity of the Filter Envelope, basically, the more you turn the velocity sensitive knob up, the more the filter will react to the incoming signal.
Hope this helps