How do I access photos and videos from iCloud?

Yes. iCloud is not a storage in a classic way but more like a sync tool. It keeps all of your photos synced at all devices. Once the photo or video is deleted, it will be erased everywhere.

But you can still access it within 30 days in the "Recently Deleted" folder. Or, if you also deleted it from there, you can try to contact Apple Support and ask if they can recover them.

All of your photos and videos are accessible on the devices that use your Apple ID and on iCloud.com.

Yes it does.

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: https://support.apple.com/en-us/HT204264

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 Photo Library 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 be lost.

If you have iCloud Photos turned on, the photos on the device are the same as on iCloud, with the possible exception that some of them may be space-saving optimized versions if you have that option turned on. You view them in the Photos app on an iOS device. You can also use a computer and login to icloud.com and go to the Photos app there. See: https://support.apple.com/en-us/HT204264