The very first time, you need to very clearly UNMUTE yourself manually by pressing a button, even after you join a voice channel.
After that first time, joining a voice channel will enable your mic, and by default it also makes a clear sound. There is a feature that reconnects you if you were connected to a voice channel before you left (though it seems to be limited to 5m now).
But again, unless you 1. connect to a voice channel and 2. press the unmute button that first time, there is no listening happening...
You can verify all this on Chrome, since there, they have to specifically ask for your permission before accessing your microphone, so you know exactly when they do it. Open Discord on Chrome and play around, join a voice channel, unmute, and only then it will ask for access.