While extensions can be downloaded from their respective browser stores, Windows Defender for Chrome and Firefox will not work without Edge support. If it determines that the URL is not in the trusted list, it will open the website in an isolated Edge session. However, any link you clicked in the isolated session will still run in the original browser if it's a trusted website.
To use the feature, you must also install the Microsoft Windows Defender accompanying application and the browser-specific extension. Since it is still in the preview mode, only Insiders have access to it at the moment, though it will eventually be widely distributed after being tested more thoroughly.