How to find text within a file and have it search multiple subfolders

grep would only find lines matching a pattern in a file, it wouldn't change the file. You could use sed to find the pattern and make changes to the file:

sed '/\B\/foobar\b/!d' filename

would display lines matching /foobar in the file. In order to save changes to the file in-place, use the -i option.

sed -i '/\B\/foobar\b/!d' filename

You could use it with find too:

find . -type f -exec sed -i'' '/\B\/foobar\b/!d' {} \;

Assuming you are using GNU find or another find which has these options,

find . -mindepth 8 -maxdepth 8 -name '*XYZ*'

If you don't have such a find command, you can use this POSIX version:

# must contain exactly 7 slashes
find . -path '*/*/*/*/*/*/*/*' -prune -name '*XYZ*'

-path '*/*/*/*/*/*/*/*' selects paths that are at least 8 levels deep (contain at least 7 slashes, depth 0 is . itself, depth 1 is the files in the current directory), doing -prune for them will prevent find from descending into those folders that are 8 levels deep, so the 9 and above ones will never be seen.