How do I know the backup was successful and where can I see the backup file?

I currently have an iPhone 6s Plus and will be upgrading to an iPhone 15 Pro Max. Note: Since I have an older iPhone, the iOS is limited to version 12. I want to transfer ALL the data from the old iPhone to the new iPhone. It was suggested I backup the old iPhone to the iCloud and restore the data to the new iPhone. I started the backup, but it did not say the backup was successful. How do I check if the backup completed? Does the backup include the music files? Is this the best way to transfer the data or is there another method that is better? Thanks in advance.