I find it funny that desktop search engines are not mentioned with a word here. This would be the first place to start when using Linux on a desktop (but of course not a server, i.e. without any desktop environment).

These search engines are very powerful because they do deep file scanning, are based on mature frameworks such as Apache Lucene.

For KDE, this was called Nepomuk https://userbase.kde.org/Nepomuk and is nowadays called Baloo https://community.kde.org/Baloo, and can be used in fact from the command line (baloosearch)

For GNOME, this is (apparently, not using GNOME in the last years) called Tracker https://gitlab.gnome.org/GNOME/tracker