A zip file with the contents of the extension, which can be uploaded to the Chrome Web Store and addons.mozilla.org, can be created with any of the following procedures (make sure to install Go before building the extension, as it is needed during the build):
Run bash release.bash -h in order to learn how to use this command. To summarize, the command accepts the --channel and --browser flags (or their short versions -c and -b).
As an example, if you wanted to create a ZIP file of the beta-branded extension targeted for Firefox, you would run bash release.bash -c beta -b gecko.
You can also use make to build the extension. This is just a wrapper for the release.bash command.
Run make all to build the extension for all the available channels and browsers. You can also run make {target} where {target} is one of the following: chromium-stable, chromium-beta, chromium-mv3-beta, gecko-stable.
Run make clean to clean all the release files (this removes the out folder, which is where the release files are saved).
If you're developing the extension, you might want to load it into your browser without having to constantly build it after each change. In order to do that, you'll only have to manually generate the manifest each time you change the template/manifest.gjson file (or only once if you don't change it, and once every time you pull new changes to your git clone).
In order to do that, run go run generateManifest.go {browser}, where {browser} is CHROMIUM, GECKO or CHROMIUM_MV3, and this will generate the manifest.json file for the specified browser in the src directory. Now, you can load the src folder directly in the browser in order to import the extension.
Chromium is working in a new version of the manifest.json file, which also introduces some changes to the extension APIs. Eventually, all Chrome extensions will be required to have migrated to Manifest V3 (MV3), so in order to anticipate this change, a new experimental Chromium MV3 target has been added.
This target is experimental in the sense that it isn't actually used when building the extension ZIPs which are uploaded to the Chrome Web Store (MV2 is still used), and because these builds aren't currently being exhaustively tested.