chromium_os-raspberry_pi | Build your Chromium OS for Raspberry Pi 3B/3B+/4B and Pi400

Git and curl as the essential tools need to installed in the host OS, you will also need Python3 for most of the scripting work in the build process. This command also installs git's graphical front end (git gui) and revision history browser (gitk).
The depot_tools is a software package of scripts, provided by Google, to manage source code checkouts and code reviews. We need it to fetch the Chromium OS source code. Then add depot_tools directory to PATH and setup proper umask for the user who is going to perform the build. Add below lines to the file ~/.bash_profile of that user. Or if you are using a different shell, handle that accordingly. Then re-login to make above changes take effective.
Now fetch this overlay, also create symlinks in the designated place.
project-cros-pi
baseboard-rpi3
overlay-rpi3
overlay-rpi4
chipset-bcm2837
It's recommended to build chromium browser on your local setup so that your Chromium OS for Raspberry Pi could benefit from the additional functionalities like kiosk mode, you will also have the option to incorporate your own modifications. If you wish to do so, you need to prepare the necessary files prior to entering the cros_sdk.
Programs running inside the chroot will not be able to access files outside of the chroot. One way to circumvent this is to bind mount those files into a directory inside the chroot. When entering the Chromium OS chroot environment, a file named .local_mounts will be checked and directories listed in it will be bind mounted inside the chroot. All we need to do is to create this file in the right place and put necessary contents in, by using below command. Now, after entered the chroot, a /path/to/overlays directory will exist in the chroot and its content is the same as the /path/to/overlays directory in the host OS, as it actually is bind mounted from the host OS. If we don't do this, the /path/to/cros-pi/src/overlays/overlay-rpi4 symbolic link will not be accessible, as the top directory (/path/to/overlays) it points to doesn't exist in the chroot.
In the Chromium OS terminology, a board refers to a class of computer platform with distinct hardware configurations. The board will be used as a target in the process of building software packages and disk image for that specific computer platform. There are many boards exist in the Chromium OS code base. They are either development platforms or real selling hardware products running Chrome OS, such as Chromebooks you can buy from many vendors. The Chromium OS project utilises the Portage package management system from Gentoo Linux. Each board lives in its own "overlay", which holds distinct build configuration, system configurations, collection of software packages, system services, disk image customisation etc. for that board. In our case here, we created a board named "rpi4" and it refers to the Raspberry Pi 4B. And we call the overlay "overlay-rpi4", all its files are hosted in this repository. To build Chromium OS for a board, the first thing is to initialise the board from its overlay. Again, it may take 10 to over 30 minutes depends on the speed of your internet connection and disk i/o. Once it's done, a directory structure for the "rpi4" board will be created under /build/rpi4 of the chroot.
Now it time to build all software packages for the rpi4 board.
On a decent machine with 4 cores 8 threads, 16GB memory, files on regular HDD, and 100Mb broadband, it takes about 5 to 6 hours for the command to finish.
On a Workstation-grade server with AMD Threadripper 3990x CPU with 64-core 128-thread, 128GB memory and 300Mb broadband, it takes 44mins for the command to finish.
After the build_packages command finished successfully, you can start building the disk image. It may take 10 to 30 minutes, mainly depends on the speed of your disk. It will be much faster on SSD than on HDD.