Bonsai is built on the Roots project. Roots is an open source framework for creating modern Wordpress sites.
Trellis handles the deployment and IT automation.
Bedrock is a reconfigured version of Wordpress.
Sage is the Wordpress theme.
Wordpress powers nearly 40% of the web. Even the word Wordpress can trigger people, because most people who have used it have simply done it wrong. This can make for a very frustrating experience. Security issues, long load times, breaking plugins, etc.
If you were going to build a SaaS app with Wordpress, using Roots would be the only sensible way to do it.
Not only is it structured like a modern app project, but you could actually decouple Wordpress from the entire project if you wanted to swap out the backend.
I’ve never personally done this before, so it might sound easier than it is. But I know people that claim it’s pretty easy.
You can also keep Wordpress in, build a custom backend, or handle all the operations from the front-end, and most users would never know it was built on Wordpress.
The reason I like working in this stack it allows people to easily become a full-stack developer. It allows one person to build amazing apps, backed by the power of a huge community and open source projects.