Isn't the top right search box used to quickly filter (search) for files?
I expected to see only files starting with q_prod and yet it doesn't show any.
Either my expectations are incorrect or I have a setting incorrect or I'm using it incorrectly.
Use the Filter Bar (* key, i.e. Shift+8) if you want to filter the current folder.
The search field at the top right will search recursively (i.e. including sub-folders) and what you type in there is sent to Windows Search which then does whatever it wants with the query. We just display the results. You probably need a wildcard in there to make it work, or it's spending a lot of time doing unindexed content searches, which can happen without being explicit about only searching names.
There's also Tools > Find Files, which uses Opus's own find functionality and is easier to use, as you don't have to learn the cryptic and badly documented Windows Search syntax (but the disadvantage is it takes up more screen space).