It’s easier to maintain, if we keep them separate in their own files.
For responsiveness, it makes no difference if they are kept in a single file or separate files.
To find any code, you can use your code editor’s search function, it will find any code in any file.
If you need to minify and combine them, you can find a suitable free plugin from WordPress.org
Keep the file loading sequence correct when you minify and combine them or it will cause issues.