If I reset my phone will it delete my files data on iCloud

No, provided you use Erase all content and settings, the icloud data is fine.

Data only on the phone needs to be backed up first

(iCloud data such as Contacts, Reminders, iCloud Data is not backed up on the backup files)

If you delete any photo, file or Note manually then yes it's gone

If by reset you mean erase the phone, it will delete the iCloud data not at first but when it backs up again to iCloud after you erase the phone. However you can restore (not erase) the phone using the iCloud backup and not lose anything. You can also backup your phone to a computer, Mac or PC, and not lose that backup (what I recommend).

If I reset my phone will it delete my files data on iCloud

Not if you follow the instructions in the Apple Support document above your name to log out of iCloud.....before....you erase/reset the phone.

If you don't, then the next iCloud will pick up the erase on the phone and backup that action, so your previous backup will disappear.