Voice activity is the default: your mic opens on its own when you talk and closes when you stop. Push to talk is the opposite: your mic only opens while you hold a key.
How to choose
Open the gear → Voice and video tab → Input mode.
Tick Voice activity or Push to talk.
On voice activity, the automatic sensitivity comes on. If Concordia is opening on the noise of the house, turn the automatic mode off: a bar appears with your live level and a marker. Drag the marker to a point above the noise and below your voice.
On push to talk, click Choose a key and press the key you want. There are three ready suggestions next to it. Some combinations are refused because they are already Concordia or browser shortcuts.
If you like, turn on the beeps for opening and releasing, on the Sounds and notices tab.
The browser limit
The fix: the Windows app. There, the key works with any program in front.
Screenshot coming soon
The Input mode section of Settings, with Push to talk ticked, the chosen key and the three suggestions.