Privilege separation is one of the cornerstones of OS security. One way to enforce privilege separation on Linux and macOS systems is to use a dedicated user account for every service. Traditionally, these accounts are known as daemon users. You may also think of them as service accounts.
You should never reuse an account of this type for anything else, it should be dedicated to its initial purpose. Such a user doesn't need a password, unless you want to debug something by running a shell under the same account.
All of the above is true for the service account created during the installation of SFTPPlus. This OS user is typically named sftpplus, and is used to give SFTPPlus processes the minimum required permissions to run and do their job. Even if authenticating OS accounts through SFTPPlus is a requirement, and the server is started under the root superuser, the elevated rights are dropped as soon as possible. Most SFTPPlus processes only run as the dedicated service account, typically named sftpplus.
This dedicated OS user is not visible in the Web Console of SFTPPlus. It should never be configured through its services, even when it's possible to authenticate operating system accounts. There is nothing to worry about managing this dedicated service account, it's created when installing SFTPPlus, and removed when uninstalling it.