There are two kinds of “aftertouch”.
The most common is “channel pressure”. It is not a property of individual notes, but rather the MIDI channel on which your device is sending note on/off events. It is typically sent when you depress a key, do not release it fully but vary the pressure until the final release. It rarely has anything to with volume in a software or hardware synth, though that does depend entirely on the synth. More commonly it gets mapped to some “interesting” sound parameter such as a filter cutoff. Often, it does nothing at all by default.
The other kind is polyphonic pressure. This is sent individually for each key (note). There are not many keyboard controllers that can generate this - it is currently more typically found on pad controllers like the Ableton Push 2 and various Novation pads. This rarely maps to anything by default in a synth, but it is normally possible to set up some connection in the synth that will make varying the pressure change some parameter. The nice thing with polyphonic pressure is that it affects each note independently.
I think you’re also asking a question about using a pitch bender, but it’s quite clear what that question is.