MIUI is described as 'Stock and aftermarket firmware for smartphones and tablet computers based on Android. MIUI, developed by Xiaomi, includes various features such as theming support. It is updated every Friday based on the feedback from its test users' and is a very popular operating system in the os & utilities category. There are more than 25 alternatives to MIUI for a variety of platforms, including Android, Linux, Android Tablet, Linux Mobile and HarmonyOS apps. The best MIUI alternative is GrapheneOS, which is both free and Open Source. Other great apps like MIUI are LineageOS, /e/, postmarketOS and CalyxOS.
I like LineageOS because to runs the latest android 7.0 and it is clean no bloatware.