Download the Mi Heart Rate app from the app store
Open the app and click on the menu icon
Click on the Apple Health switch
Allow Mi HR to access and write heart rate data by clicking on the switch, then click "Allow" at the top right corner.
Open the Mi Fit app and tap the profile button on the lower right corner
Scroll down and tap Add accounts. Mi Fit will offer to automatically pair with WeChat and Google Fit.
Click on Google Fit and confirm the sync.