Mobian is described as 'open-source project aimed at bringing Debian GNU/Linux to mobile devices'. There are more than 25 alternatives to Mobian for a variety of platforms, including Android, Linux, Android Tablet, Self-Hosted solutions and VMware Workstation. The best alternative is LineageOS, which is both free and Open Source. Other great apps like Mobian are Android (Free, Open Source), Ubuntu Touch (Free, Open Source), Android-x86 (Free, Open Source) and /e/ (Free, Open Source).
MIUI is a 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.
postmarketOS (pmOS), is a touch-optimized, pre-configured Alpine Linux that can be installed on smartphones and other mobile devices. The project is at very early stages of development and is not usable for most users yet. We develop a sustainable mobile OS
Slim ROM is super-lightweight Android ROM for Samsung Galaxy phones, that's aimed at advanced users. Slim is designed to trim the fat down to the bare essentials and give you a fast and clean Android installation.