How can I see what I have stored in my ICloud on my iPhone 8plus?

Whatever is already on your phone is in iCloud. iCloud is a syncing service and not off device storage. If you delete something on your phone that syncs with iCloud, it is removed from iCloud as well. Therefore, there is no need to see what is in iCloud because it is already on your device.

Log in to https://www.icloud.com with your Apple ID credentials. You have everything that is synced from your iPhone to iCloud.