The \ is treated literally.
\xxx
Will only match files and folders that start with xxx.
This will also match files and folders in paths that have a folder that starts with xxx.
C:\abc
\xxx123 matches.
C:\abc\12xxx3 does not match.
C:
\xxx123\abc matches.
C:\12xxx3\abc does not match.
\ xxx
Is two separate terms: \ AND xxx.
\ will match all files and folders except for the roots as they have no paths.
xxx will match only filenames, not the whole paths.
If you want to find xxx anywhere in the path, please enable match path from the search menu, or use the path: modifier:
You can also use wildcards to find xxx anywhere in the path: