How do I only backup data for specific apps on iOS?

Source:

Choose which apps to back up

Many iOS apps automatically back up to iCloud after you install them. You can change which apps back up and remove existing backups from your storage. Use these steps on your iPhone, iPad, or iPod touch.

iOS 10.3 or later:

1. Go to Settings > [your name] > iCloud.
2. If you use iOS 11, tap Manage Storage > Backups. If you use iOS 10.3, tap iCloud Storage > Manage Storage.
3. Tap the name of the device that you're using.
4. Under Choose Data to Back Up, turn off any apps that you don't want to back up.
5. Choose Turn Off & Delete.

iOS 10.2 or earlier:

1. Go to Settings > iCloud > Storage > Manage Storage.
2. Tap the name of the device that you're using.
3. Under Backup Options, turn off any apps that you don't want to back up.
4. Choose Turn Off & Delete.

When you confirm that you want to Turn Off & Delete an app, it turns off iCloud Backup for that app and removes all of its information from iCloud.

Some apps, like Voice Memos, are always backed up and you can't turn them off.

I know it doesn't related to icloud, but if you have jailbreaked device, you can use "Apps Manager" tweak (https://www.tigisoftware.com/default/?page_id=259) to locally backup/restore data by app. It works fine even on iOS 10 and 11