How do I know if ALL my pictures and videos are stored in my iCloud from all devices. Does it start when you set up iCloud going forward or does it capture everything you have on that particular device?

How do I know if ALL my pictures and videos are stored in my iCloud from all devices. Does it start when you set up iCloud going forward or does it capture everything you have on that particular device? I have two phones. One for work with only a particular amount of storage and the other personal . I have 2 TB of storage in iCloud. The work phone is telling me Im out of storage and to delete large files, which apparently my videos. But if I do that I was afraid of losing them or deleting in ICOUD also. So I started sending them to my personal phone. I see them trying to sync- but they are not. If they sync will they appear as the latest videos or go to the date of which they came from. Getting so confused. I need to make space on my work phone 64GB but afraid to delete as it may delete on all devices since everything is in the cloud. How do I free up space? Sorry if Im not making sense. I don't want to lose photos or videos by removing them.