I'm not sure all my photos transferred to my new Galaxy S9.

It seems my photos on my old phone stored in so many places,..internal, sd card, cloud this and cloud that. Now i have a new S9 and it keeps suggesting a photo backup and I'm afraid to back up my photos, Here's my fear: if some are missing on my new phone and I back it up where 'missing' photos still are, won't it only store what I back up and I'll lose the others forever? As you can tell, this is all confusing for my senior brain. Any suggestions?

No, backing up pictures is kind of "one way" - what's on the phone gets sent to the server. If there's a picture not on the phone, it just doesn't get sent.

Welcome to Android Central! If you want a simple cloud backup system that you hardly have to think about, take advantage of Google Photos backup: https://support.google.com/photos/an...DAndroid&hl=en. If you select the High Quality setting, you get unlimited backup storage.