where does my MacBook store backups of my iPhone and iPad?

Go to iTunes. Look under iTunes>Preferences>Devices

It should have all the backups listed by the name of the device with time of last backup, and maybe additional ones that have been archived. From there you can right-click on any particular backup and one of your options is "Show in Finder". This is the preferred way because the actual name of the backup folder is some inscrutable, long set of random alphanumeric characters.

You can also archive any existing backup such that they don't get overwritten by new backups. For anyone looking at the Windows version of iTunes, there is no way to do this in that version.