There are different ways to fill the Apple cloud storage. You can fill the "iCloud Drive" folder on the Mac, deposit iOS backups or allow apps to use the online storage. Especially the last ones are insidious because they are rarely controlled due to the relatively hidden access. But a look into the memory is worthwhile and should be done on a regular basis, for example, to remove data from apps that have long been deleted and to make room for something new.
How to delete iCloud data on iPhone & iPad
Open the Settings app on your iPhone or iPad. In the Settings browser, tap the top of your Apple ID, then select iCloud. Under the heading "Storage" you can now see your used iCloud data. Tap on "Memory". Loading the data may take a moment.
You will now see your memory usage again and also have the option to change your memory plan here. A little further down, all apps are visible. Tap an app, then select "Delete data." In some cases, you have the option to delete all data or individual elements. The deletion process must always be confirmed.
Once you have deleted all the data you want, just go back to the last screen. Repeat the process as needed for other apps.