How my photo convert to iCloud
Send to my picture in iCloud
Switch on iCloud Photos in photos settings.
Note the following:
1 - you will need to pay for a plan with enough space for all your photos.
2 - if you delete photos from your phone, they will also be deleted in iCloud (iCloud is a sync service, not a backup service)
If you are short of space on your phone, then you can turn on "optimise phone storage" and it will only store small previews of photos on your phone, and the full size originals will only be in iCloud.