How I enabled "Google Play Services for AR" on my Mi A2
Google Play Services for AR (previously ARcore) is Google’s platform for AR apps. It’s supported on most developer friendly Android devices. However, Mi A2, despite being an Android One device, haven’t gotten ARcore support yet. 
But there’s a way to enable ARcore unofficially on most Android phones with capable hardware.
Step 1: Enable Camera2 API
Follow this guide to temporarily root your phone and enable Camera2 API: Enable Camera2 API and Download Google Camera Port for Xiaomi Mi A2/A2 Lite
Step 2: Install Google Camera (Optional)
An added benefit of enabling Camera2 API is access to Google Camera. The official Google Camera on Play Store is available only for Pixel devices. So, you’ll have to sideload a mod of Google Camera for your device. Try one of the apk builds from these Google Camera mods optimized for Mi A2: Google Camera mods by the_dise.
Step 3: Enable Google Play Services for AR (ARcore)
- Follow this XDA Post
- See this GitHub issue for more info: An UNSUPPORTED holiday gift from the ARcore team
- Sideload Google Play Services for AR (ARcore) APK from here
Step 4: Install Playground (Optional)
If you installed Google Camera in Step 2, you can install Google Playground addon for it and play with AR stickers. Sideload Playground mod from here.
Credits & Sources
Featured image derived from post by Pranob Mehrotra on XDA