I've been using bootstrap 5 in my project and it works pretty well. I think the docs outline how to enable it for your project (Project pane -> Right-click on Project -> Manage Javascript Libraries?).
One area where it can be used (and where I use it the most often) is to add bootstrap classes to the various controls where needed. For example, to add a bit of padding at the start of the editor in the TWebEdit control, just add 'ps-1' to the ElementClassName for that control.
This handy little reference helps with figuring out what the bootstrap classes are if you're not already familiar with them:
Bootstrap 5 Cheat Sheet
I think pretty much all of the Web Core components have ElementClassName properties or related properties that can readily make use of bootstrap classes. There are many other areas where they can be used as well, depending on what it is you are trying to do.