OmniROM was a very famous custom ROM in the past and it may surprise you to know that it is still under development. This project is once again in the headlines for a new port by Max Weninger which brings the software to Raspberry Pi 4.
Though Raspberry Pi uses the same ARM architecture as most Android devices, porting of Android to Pi has always been a very tricky thing to do. This is mainly due to the missing drivers. Various attempts have been made to bring Android to different Pi models over the years but the new OmniROM port appears to be one of the most impressive builds yet. It has been based on Android 11 and has been designed for Raspberry Pi 4. if you are using older models you will have to stick with other operating systems.
The ROM runs Android in the tablet mode not like an Android TV device. That is a better fit for Pi which is used as productivity and learning PC. Weninger noted a forum reply which says, I really don’t think a raspi is a well equipped device to be used for ATV when you [can get an] ATV USB stick from e.g. Xiaomi for about 40E that simply works without issues and has official Google support.”
the forum thread says that hardware-accelerated video playback is not present and Mesa support for 3D graphics is a work in progress. Android typically does not work well when the booting from slow storage like SD cards but the ROM supports running from either USB or SD.