To find details about the last logged-on user on Windows, you can query the registry:
Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI" | Select-Object -Property LastLo*, Idle*
The result looks similar to this:
LastLoggedOnDisplayName : Tobias Weltner
LastLoggedOnProvider : {D6886603-9D2F-4EB2-B667-1971041FA96B}
LastLoggedOnSAMUser : .\tobia
LastLoggedOnUser : .\tobia
LastLoggedOnUserSID : S-1-5-21-2770831484-2260150476-2133527644-1001
IdleTime : 62486093
Likewise, this line returns all the user profiles registered in the Windows registry:
Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\profilelist\*' | Select-Object -Property ProfileImagePath, FullProfile
