Files has no way to search with in a directory while Nautilus does. How to I replace Files with Nautilus?
Files does have a search facility - you just start typing or press
<Ctrl>F and type into the pathbar (which doubles as search entry). The first results returned are those found in the current directory. It also returns (grouped separately) results found in subfolders of the current folder, in bookmarks and in recently used.
However, if you wish you can install Nautilus from the AppCenter.