Project Sandcastle is described as ': Android for the iPhone'. There are more than 10 alternatives to Project Sandcastle for a variety of platforms, including Android, Linux, Self-Hosted solutions, Android Tablet and iPad. The best alternative is LineageOS, which is both free and Open Source. Other great apps like Project Sandcastle are Android (Free, Open Source), Ubuntu Touch (Free, Open Source), /e/ (Free, Open Source) and Replicant (Free, Open Source).
A free and open-source operating system for smartphones and tablet computers, based on the Android mobile platform. It is the successor to the highly popular custom ROM CyanogenMod, from which it was forked in December 2016 when Cyanogen Inc.
Ubuntu Touch is a mobile version of the Ubuntu operating system for mobile devices. It is adapted to run naturally in a mobile, touch-screen environment but is also capable of functioning as a desktop computer while in "desktop mode".
/e/ is a fork of LineageOS that aims to be completely free of everything Google. Not only that, but there are a number of goals the developer has in mind for /e/ that more people feel are vital to a mobile operating system.
iOS is a mobile operating system developed by Apple Inc. and distributed exclusively for Apple hardware. It is the operating system that powers iPhone, iPad and iPod Touch. Each main version of iOS / iPhone OS has introduced a big, main feature of the new version.
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.