If you want to install the google’s official app, you can use this workflow with microG: https://xdaforums.com/t/how-to-use-digital-wellbeing-on-miui-10-android-9.3891718/

There are other alternatives to the ones they recommend in that post. Basically there 2 steps:

• Get the wellbeing app from somewhere. If you use Aurora Store, you can download it from this link: store link
• Create a shortcut for it. I use AppManager where you can also create shortcuts for activities