While Xiaomi now makes some of the best Android phones, it started out with MIUI, offering a software interface with a design inspired by iOS but with several unique additions, including a robust theming engine and useful pre-installed apps. The first time I used MIUI was on a Nexus 4 all the way back in 2013; MIUI 5 was very different to the default Jelly Bean interface, offering an astounding level of customization (for the time), permission management, live icons, a music player that was better than what you'd get on most phones, and robust security features.
Of course, Xiaomi was already making phones by this point, but they were limited to its home market. MIUI allowed Xiaomi to gauge interest in its software interface outside of China, and the brand amassed a decent userbase in a short amount of time, paving the way for an entry into the Indian market, where it debuted its first phone — the Mi 3 — back in 2014.
MIUI went through a lot of changes over the last seven years. MIUI 6 introduced a new UI with bright colors and updates to notification management. MIUI 7 came with better customizability and a more optimized UI, and had a data-saver mode that works system-wide. MIUI 8 saw the debut of Dual Apps, video editor for the gallery, new notification pane, and spam call protection.
Xiaomi further refined the notification pane with MIUI 9, added new editing features in the Gallery to remove background elements, several under-the-hood optimizations, and rolled out Mi Video. MIUI 10 introduced a few features that are still present today: rounded icons for the quick settings tiles and a brightness slider, vertical recents menu with a card-based UI, and picture-in-picture mode. MIUI 11 introduced a system-wide dark mode, AOD, cleaner design with modern icons, and new battery-saving modes.
The number of features added over the years meant that the interface was bloated by the time MIUI 10 was introduced. Xiaomi has a limit on hardware margins — that's why its phones are so affordable — so it instead turned to ads within MIUI and pre-installed apps as a way to bolster profits from its phones, and that made MIUI 10 far from enjoyable. That pattern continued with MIUI 11; the Redmi Note 8 Pro in particular had more bloatware than any other phone I've used to date, and the incessant ads led to an outcry from users in India and other Asian markets, where Xiaomi fielded an ad-based model for MIUI.