If I buy iCloud storage, will the storage go to just my photos or will it go to my apps?

Purchased iCloud storage is used for any and all things that store data on iCloud. That includes photos, files, videos, contacts, notes, calendar, email etc... as well as any apps that save data and files to iCloud.

As mentioned, it has no effect on your device's local storage capacity.

iCloud storage does not increase the storage space on your phone. It increases the online space used to sync your data to other Apple devices and making backups of your phone.

The difference between phone storage and iCloud storage -

What's the difference between device storage and iCloud storage? - Apple Support