There are many ways to call the Google Assistant, I have a dedicated button on the left in the Mi Mix 3. You can also use your voice saying "Ok Google" or "Hey Google." For this purpose, the application tells 4 to repeat "ok Google" in order to recognize the voice of the owner of the smartphone. Despite this, I have the option of Voice Match in my Mi Mix 3 and so it can not be activated, a big damage severely limits the functionality. For me, the most important task of Google Assistant is to control the phone when I am driving a car, it is a pity that I need to manually call it.
Call the Google Assistant using the buttons on the Xiaomi smartphones
If you also have a problem with Voice Match and an automatic call to the assistant and you do not have a dedicated button like me in Xiaomi Mi Mix 3 is nothing lost. Xiaomi has implemented a function in MIUI 10.21 that uses this button for switching the smartphone on / off. If you have a system in the new version, just go to settings -> additional settings -> shortcuts and gestures -> and at the bottom will be Google Assistant - "Hold down the power button for half a second to launch the Google Assistant".
If you have not used the Google Assistant yet, check it, I really recommend it. You can quickly answer an SMS, set up a navigation or add an event to the calendar next Tuesday at 11. In my opinion, Google has the potential to make their virtual helper the clear leader from the competition. All thanks to google search engine and billions of searches that are searched in it.