The structure of a project file set can vary according to the purposes of a project, but best practices suggests keeping .R files in a project subdirectory called R/. If you want to find a function within a file and don't know where it is, use the Go to file/function search bar near the top of the RStudio IDE. Start typing the function name and once the function is identified you can hit enter or tab to navigate to the file and even the line in the file where that function definition begins.
Hope this is helpful.