iCloud storage

Your iCloud backup will always include all of the information listed in "what does iCloud backup." Whether the backup includes some items (photos, etc.) is dependent on whether you use iCloud photos and other services. For example, if you use iCloud photos, your pictures will not be in your backup as they are already synced into the cloud.

The purpose of the backup is to be able to restore your device pretty much as is should you need to erase your current device, get a new device, etc. If you have significant information on your device, you'll have larger backups requiring more space in iCloud.

You can, of course, backup your device to your computer instead. https://support.apple.com/en-us/HT203977

There is only one iCloud. You have 5GB of free space and can purchase additional storage as needed. iCloud syncs your content across your devices so that photos, documents, and data remain synchronized between iPhones, iPads, and Mac. Additionally, some content is available at iCloud.com. When you backup to iCloud, you're simply storing your backup in that location (you could also backup to a computer). The guide below is the intro which should help.

https://support.apple.com/guide/icloud/introduction-to-icloud-mm74e822f6de/icloud

See What does iCloud back up? to learn about the data being saved into iCloud.

You may need more than 50GB depending on how much data you have to back up.

You can access any of your files at iCloud.com or on your connected device if you sync with iCloud.

If you go to Settings > Apple ID > iCloud you'll see a list of items that you can sync with iCloud. Some of those items are then available at iCloud.com (eg. photos, files, reminders, notes). Other items that you can toggle on/off sync data across devices (Messages, stocks, wallet, etc.) and are not directly accessible.

That's different from a backup; which backs up your phone contents but generally ignores items already in iCloud.