https://getbootstrap.com/docs/4.3/components/alerts/
Perhaps a gallery of example websites & apps could help a lot. That was the first thing I was looking for. Couldn't find any, so I had to drill into the docs in hopes of finding some case-by-case examples of each UI element. It really helps me to get going on frontend design, especially on a new framework, when I can see a completed sample that has some similarities to what I want layout-wise in my design.
The most direct Fluent Design System library to consider compared to Bootstrap would be Fabric UI which covers Web, iOS, and Android:
https://developer.microsoft.com/en-us/fabric#/
Up until recently Fabric UI didn't always match Fluent Design goals, but they've aligned to Fluent a lot more over recent releases.
Though, generally the "closest to homebase" implementation of Fluent Design is often regarded to be the WinUI library that is the basis for most "modern" UI controls in Windows, in particular its where you see the most of some of the exotic materials Fluent Design likes such as the "acrylic" backgrounds that a lot of people really associate with Fluent:
No matter what design they pull over their software, it will not make genuinely substandard software cool.