Why does Bridge offer Spotlight or Bridge as search options?

If Bridge uses Spotlight search, it has to take whatever Spotlight hands it. You can customize what Spotlight can and can’t search, using the Spotlight preferences pane, and there may be other capabilities or restrictions that Apple has given it.

Because the Bridge search engine is by Adobe, it isn’t going to work exactly the same way as the OS search engines in macOS or Windows.

Update: I think the answer is this one in the help file:

Bridge: Perform a Quick Search

Quote:

The Adobe Bridge engine searches filenames and keywords. Operating system engines look for filenames, folder names, and image keywords. Adobe Bridge search looks within the currently selected folder and all subfolders, including My Computer (Windows) and Computer (Mac OS). Operating system search engines look in the currently selected folder or in My Computer (Windows) and Computer (Mac OS).