Things are reduced as much as you use them, the same applies to smartphones. The more apps, photos and videos you have, the more internal storage is consumed and the less space you have. That's why we sometimes have to sort out unnecessary things and recover space.
iCloud storage is basically the same, and the 5 GB space that was initially allocated and can be used free of charge will eventually be used up. Unlike the built-in storage, you can increase it by additional purchase (upgrade) such as 130 yen / month at 50 giga, 400 yen / month at 200 giga, but it is basically to recover unnecessary things by organizing unnecessary things.
When organizing your iCloud storage, start by finding out which apps/services you use most. Open "Settings" -> "Apple ID" -> "iCloud" and tap "Manage storage". Check the available capacity (upper limit) and free space in the band graph displayed at the top of the screen, and then check the apps/services arranged in descending order of consumption.
The apps/services displayed on this screen can be broadly classified into three types. One is a type that can be deleted for each saved document, such as "Keynote", "Numbers", and "iCloud Drive". You can quickly recover the iCloud storage space by deciding whether or not you need the document name and deleting the unnecessary ones.
The second is a type that deletes all documents/data saved by apps/services. You can't choose individual data, but if your app is just data that isn't that important, it won't hurt. "Memo", "reminder", and "voice memo" correspond to this type.
The third is a type that can not do anything on this screen. "Email" can check the consumed capacity, but there is no function to delete individual emails. While checking the contents with the "email" app, you will have no choice but to judge whether it is necessary or unnecessary. You can also batch delete photos/videos uploaded to iCloud in "Photos", but you can't see the images, so it takes a lot of courage to work on this screen.
Easy explanation of operation procedure