If I've only used iCloud on my iPhone for a backup and I go to turn on iCloud on my iMac, will it automatically start populating or storing things without a specific prompt by me? I don't want it to right-off-the-bat.

If you turn on iCloud Drive, without turning on the Desktop & Documents option, it will only create an iCloud Drive folder on your Mac. If you add files to that folder it will sync them with iCloud (and with your iPhone if you have that option turned on in your phone). Do NOT turn on the Desktop & Documents option until you fully understand it and decide that you want it. It will automatically sync all of your Desktop and your Documents folder.

See: Set up iCloud Drive - Apple Support

Add your Desktop and Documents files to iCloud Drive - Apple Support

Work with folders and files in iCloud Drive - Apple Support

iCloud Photos is a separate option: Set up and use iCloud Photos - Apple Support

There are other iCloud syncing options (e.g., Contacts, Calendars, Notes, etc) in Apple menu > System Preferences.