Much of the software used by Google is based on open source elements that Google then customizes with its services, and Chrome is no exception. Chromium is the open source version of Chrome, before the company added its various services and integrations.
Because it’s open source, Chromium is used as the foundation for other leading browsers, including privacy-focused Microsoft Edge, Opera, Vivaldi and Brave. Basing a browser on Chromium offers many of the same benefits that Google experienced when it decided to base early versions of Chrome on WebKit, including compatibility. This allows browser developers to focus on features and functionality, rather than reinventing the rendering engine wheel.
Microsoft Edge and Opera are two good examples. Both browsers were initially based on their own rendering engines. Ongoing compatibility issues, along with lower performance, caused both companies to adopt Chromium as the basis for their browsers, allowing them to benefit from the work already done by Google.
In addition to Edge, Opera, and other Chrome-based browsers, users can download Chromium directly, essentially enjoying a Chrome-style browser without Google add-ons.