Dolphin uses Baloo search, but there is no obvious way to open configuration dialog for Baloo search engine. The workaround is to run "kcmshell5 kcm_baloofile" in a terminal. Reproducible: Always
Should we add some info where to configure to https://docs.kde.org/trunk5/en/applications/dolphin/quick-tips.html#finding-searching-in-file
I thin it's pretty clear. System Settings > Search > File Search
Actually Dolphin *could* embed the very same KCM in its Setting Dialog.
That's true, it could, but that's really a nice-to-have, not a critical blocker, since the UI is already available elsewhere from a logical and expected location.