Viewing and managing my content in iCloud

On an iPhone, iCloud is designed to be viewed by using the apps on the device (e.g., the Files app for iCloud Drive, the Photos app for iCloud Photos, the Notes app for notes, etc.). A Mac is similar except that iCloud Drive is accessed via the iCloud Drive folder in Finder.

You can also use an internet browser and log into icloud.com. What you can access via a browser varies by device. See the information in the following Apple article: Which iCloud.com features are available in the browser on your device - Apple Support

Also note that while you can view synced data as described above, it is not possible to view the contents of an iCloud Backup of an iOS device. You can only use the backup to do a full restore of a device or to setup a new device.

Apple also has an article about managing iCloud storage. See: Manage your iCloud storage - Apple Support

You are clicking on compose icon that is within mail application that's the reason mail compose window opens , click on cancel .

Regarding , photos you have selected all photos from library ( used command and A keys ) , click on the drop down arrow and click on hide View your photos and videos on iCloud.com - Apple Support

Hide photos or videos

You can hide a photo or video if you want to store it in iCloud Photos but do not want it to appear in your library. You can view hidden photos or videos in the Hidden album.

If you hide a photo in your iCloud Shared Photo Library, it is hidden for all participants.

1. In Photos on iCloud.com, select the photos or videos you want to hide, or double-click a single photo or video.
2. Click , then click Hide.

If you want a photo or video to appear in your library again, go to the Hidden album, select the photo or video, click , then click Unhide.

That's the reason when you click on library no photos are shown .

Sign in with your Apple ID and password with account credentials from a web browser at www.icloud.com , click on the app say mail , the contents of it say mails in inbox , sent can be viewed , same way for other apps click on the app and view photos .

Scroll down you have to click over customise home page .

If you click over mail icon .

It can be edited .

See the article Customize the homepage on iCloud.com - Apple Support

Also if you want to delete some mails , notes , photos , documents in iCloud Drive it can be deleted .

See the link for iCloud user guide iCloud User Guide - Apple Support

You cannot view or access iCloud Backups of iPhones or iPads by using an internet browser and going to www.icloud.com.

It is possible that your iCloud storage space is full because of Backups.

You can use the following to get more information about what is stored in iCloud and to get instructions for deleting any backups of devices that you no longer have. See: Manage your iCloud storage - Apple Support