there’s only one velocity I know: a number which is sent with ‘NoteOn’ MIDI command.
yes, it is determined by how hard you hit a key.
no, you’re wrong, that this value can’t be controlled with an envelope (at least many other popular DAW’s can do that somehow)
there’s simple algorithm: when note is about to play (‘NoteOn’ event is coming) then value for velocity is a) taken from the envelope graph; b )taken from the current note velocity/default velocity and multiplied by coefficient determined by envelope.
second method is more popular. In fact this envelope should be called ‘velocity scale’ instead of ‘velocity’.
velocity envelope typically useful to get overall volume changes (crescendo-diminuendo), while details are defined by default velocity (from keyboard/entered manually/etc).
for now I use ‘Veloscaler’ plugin to simulate this. I explained it here:
https://forum.renoise.com/t/where-is-note-velocity-graph-automation/25931