Starting with Windows 10 May 2019 Update version 1903, a new voice activation privacy setting will let you choose whether apps can listen for a voice keyword and then continue listening to the microphone after a keyword is detected.
Starting with Windows 10 May 2020 Update version 2004, you can choose what happens when you press the headset button for which voice activation app will be used by default.
Turn on (default) this setting to have your headset use whatever voice activation app you most recently used (aka; last used).
Turn off this setting to choose the default app that your headset will use.
This tutorial will show you how to change which voice activation app will be used by default when you press the headset button for your account in Windows 10.