I have two questions about simple searches in Directory Opus. By simple, I am referring to entering the search term in the small box in the top-right corner, not by using Tools > Find Files (Ctrl+f).
How do you search among filenames only (ie limit results to only those files in which the search term is part of the filename)?
How do you include or exclude search results from subfolders?
Can searches be narrowed in those ways when using the upper-right search box or is a Ctrl+f search required in those cases?
The top right field passes whatever you type into it to Windows Search. Syntax is the same as in File Explorer, although Microsoft haven't documented it very thoroughly. If you push F1 while in the field I think it should take you to help for it.
Prefix search terms with filename: to make them only search in the names.
If you want to search only names and exclude subfolders, I would not use Windows Search at all. Instead, push * to open the Filter Bar, and type what you want in there. (If you turn on Flat View, you can also use it to filter in subfolders.)