I don't back up my iPhone that often. It seems as if every time I try to capture a backup on iTunes, the option to start a backup has moved somewhere else. Perhaps Apple designers are trying to make life difficult for users or perhaps they are trying to get everyone to switch to iCloud backup and pay them for more iCloud space required for backups or perhaps ?
In any case, I just discovered that I can back up to iTunes using File > Devices > Back Up on iTunes.
I submitted this question after reviewing the pages at https://support.apple.com/en-us/HT203977 and https://support.apple.com/en-us/HT203271
I reviewed the list of questions submitted regarding the article at https://discussions.apple.com/article/HT203271?page=1&perPage=60 and did not find a question which seemed applicable.
Please note that the page at https://support.apple.com/en-us/HT203977 does not explain how to start a backup on iTunes, making the help page ludicrously unhelpful! The page says only, "5. When the process ends, you can see if the backup finished successfully." which is useless if one is not told how to start the backup!