In my case I wanted to remove a 21GB backup of an iPhone that was stored on a MacBook Air with macOS Monterey installed. For some reason the iPhone would not connect to the MacBook and only flashed the "trust this device" prompt without leaving me time to accept it.
To locate the backups using the file system, navigate to the following location and delete the folder there
~/Library/Application Support/MobileSync/Backup/
For more info and help in other cases, check the provided source below.