If I press the "Off" switch for a second, the Google Assistant will respond, and only then will the phone shut down. How can I deactivate this on my mobile phone so that the Google Assistant no longer reports?
Deactivating the google assistant is not possible, I think, but the google assistant is actually placed on the home button, the only thing I can imagine is that you have activated something somewhere in the settings, e.g. Conditional aids, that the Google assistant now also reports to the button
And where in the settings can I change this?
Look in the Google settings to see if you can possibly deactivate the Google Assistant for your account, otherwise look in the device's user help and if nothing is to be found there
If the google assistant should also be able to be deactivated via the assistant itself, start the assistant with "ok / hey Google" or via the home button then on "assistant devices" and it should be possible to deactivate it for the desired device, mobile phone or tablet
Where in the settings of the phone can I change it…
I didn't say that you can change it there, but you might find something there, but I've never dealt with that so much. Have you started the Google Assistant and then tried the settings?