How can I check what is stored in my iCloud account?

iCloud is a syncing service. If you delete data that is synced with iCloud from your iPhone it will also be deleted from iCloud. Typically the items that take up the most space are photos and videos and the following may help with that. Note the Optimize Storage information in particular.

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will eventually be lost.

That's not how iCloud works. iCloud is more of a synching service than a storage. For exemple, if you use iCloud Photos service, the files are in iCloud but you see them on your camera roll, and when you want to see the file in full quality or share it somehow, file will first be downloaded to your phone and then shared. The idea is to help you save storage on your device.

But if you just want to delete it, once you delete it on your phone it will also be deleted from iCloud.

This also happens with files in Files app.

Third party apps have their own way of storing files, usually using the internal storage of the device for it, not iCloud.

Manage storage on iPhone - Apple Support

Set up and use iCloud Photos - Apple Support

Manage your photo and video storage - Apple Support

some other tips How to free up storage space on iPhone? - Apple Community

The information I posted is correct and as I noted, iCloud can help you reduce the storage needed on your phone if you turn on the Optimize Storage option for Photos. See the following from Set up and use iCloud Photos - Apple Support

If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them.