How to back up iPhone to iCloud?

See the following Apple Support article for information on backing up an iPhone to iCloud: How to back up your iPhone or iPad with iCloud - Apple Support

Also note that another option is to sync data with iCloud. Syncing happens automatically (if turned on) when data changes and synced data can be viewed in an internet browser at www.icloud.com

Also, synced data won't be duplicated on iCloud by an iCloud Backup (if the data is synced it won't be included in a Backup). For some information about syncing, see: Set up and use iCloud Photos - Apple Support

and

Set up iCloud for Contacts on all your devices - Apple Support

and

Set up iCloud for Notes on all your devices - Apple Support

Other syncing options are at Settings > [username] > iCloud.