The area on the left in grey is called the "navigator"
You've selected the Find navigator, and you expect to see files containing text containing "readNumber".
You're searching in an area called "macOS" (selected using the popup underneath the search field. I don't know what that is - it isn't one of the pre-defined search scopes. I usually use "in Project" or "in Project & SDK".
In addition, if you look at the bottom of the Find navigator, there's a file name filter. Your search doesn't find "readNumber" at all, so this doesn't make any difference here, but I have no idea where it is searching.
Clear the file filter at the bottom and change the search scope to "project" and see if it works then.
Also, select the Project navigator, expand the project view and post the screenshot - it should looks something like this
if it doesn't, maybe we'll be able to help you out.
Incidentally, clearly Xcode is finding something, because it built and ran your program. Also, if you're new to this, why are you building for both macOS and iOS? It would be easier to start with just one.