MIUI stands for Mobile Internet User Interface. Where the MIUI software was developed by a Chinese supplier, namely Xiaomi, to integrate the user interface of the Android system running on their product.
As is known, the 'Pure' Android system does not include a screen like in several smartphone brands that carry the respective themes. Samsung, OPPO, Coolpad, Asus and others have their own UI theme for their features and also allow users to easily interact with their mobile product. If you want to see what Android looks like, it's actually very easy to find, namely AndroidOne, which is currently manufactured by various smartphone vendors at low prices.
Going back to MIUI, maybe you once had a Samsung branded Android phone famous for its TouchWiz theme, then borrow a friend's Xiaomi branded phone with MIUI, even if it will have the same Android version, display and the order menu on both phones is different because each manufacturer gives a different "touch" to the design and appearance.
MIUI itself is built using the programming languages Java, C and C ++, the monolithic kernel type, where the kernel is a modified version of the Linux kernel. MIUI is licensed under the GNU General Public License v3, as well as the Apache 2.0 license.