Compare the interface between android oreo 8.1 with android P 9.0 DP1 in Mi A1. the result as shown below. of course this is not real, because there is currently no rom android P for Mi A1. Only based on LineageOS 15/15.1
If you want to get same like android P, you need to install the tools:
>Latest xposed installer v3.1.5 apk ( you can download xposed-framework in app)
>latest version android P-ify.apk
>Substratum theme.apk
>Flux white.apk
>LockClean substratum mod.apk
>//Substweaks//.apk
>Round screen corners pro.apk
>Pixel launcher 2 android P.apk
>Rootex.apk
for the steps I think you already understand. so please try it yourself.