runonce.exe is a program to service the customer of types the services like any tool for an installation or a setup, It's a custom to operate any kind an installable
PS C:\> Get-ACL -Path "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce" | Format-List
Path : Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce
Owner : NT AUTHORITY\SYSTEM
Group : NT AUTHORITY\SYSTEM
Access : BUILTIN\Users Allow ReadKey
BUILTIN\Administrators Allow FullControl
NT AUTHORITY\SYSTEM Allow FullControl
CREATOR OWNER Allow FullControl
APPLICATION PACKAGE AUTHORITY\ALL APPLICATION PACKAGES Allow ReadKey
null Allow ReadKey
Audit :
Sddl : null
PS C:\>