Here are the answers to your questions.
Does the description of Administrators (computer/domain) imply all privileges of WSS_Admin_WPG?
No Administrators Group have full priviledges in the system and WSS_Admin_WPG have very specific privileges in the system.
These special priviledges includes rights to registry entries
"HKEY_CLASSES_ROOT\APPID\{58F1D482-A132-4297-9B8A-F8E4E600CDF6} "
"HKEY_CLASSES_ROOT\APPID\{6002D29F-1366-4523-88C1-56D59BFEF8CB} "
"HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VSS "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Office\14.0\Registration\{90120000-110D-0000-0000-0000000FF1CE}
" "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server "
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0 "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Office
Server\14.0\LoadBalancerSettings "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Office
Server\14.0\LauncherSettings "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Office Server\14.0\Search "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Shared Tools\Web Server
Extensions\14.0\Search " "HKEY_LOCAL_MACHINE\Software\Microsoft\Shared
Tools\Web Server Extensions\14.0\Secure "
"HKEY_LOCAL_MACHINE\Software\Microsoft\Shared Tools\Web Server
Extensions\14.0\WSS "
rights to file systems
"%AllUsersProfile%\Application Data\Microsoft\Sharepoint "
"C:\Inetpub\wwwroot\wss " "%ProgramFiles%\Microsoft Office
Servers\14.0 " "%ProgramFiles%\Microsoft Office
Servers\14.0\WebServices " "%ProgramFiles%\Microsoft Office
Servers\14.0\Data " "%ProgramFiles%\Microsoft Office Servers\14.0\Logs
" "%ProgramFiles%\Microsoft Office Servers\14.0\Data\Office Server "
"%windir%\System32\drivers\etc\HOSTS " "%windir%\Tasks "
"%COMMONPROGRAMFILES%Microsoft Shared\Web Server Extensions\14 "
"%COMMONPROGRAMFILES%\Microsoft Shared\Web Server
Extensions\14\ADMISAPI " "%COMMONPROGRAMFILES%\Microsoft Shared\Web
Server Extensions\14\CONFIG " "%COMMONPROGRAMFILES%\Microsoft
Shared\Web Server Extensions\14\LOGS " "%COMMONPROGRAMFILES%\Microsoft
Shared\Web Server Extensions\14\Data " "%windir%\temp "
"%windir%\System32\logfiles\SharePoint " "%systemdrive\program
files\Microsoft Office Servers\14 folder on Index servers "
Does the description of WSS_Admin_WPG (in part of "system resources used by Microsoft SharePoint Foundation") imply Sharepoint Server?*
WSS_Admin_WPG group is used by Application Pool Accounts for Central Administration and Timer Services.
What are relationship (difference) between "BUILTIN\Administrators" and "Administrators"?
The BUILTIN\Administrators group is a member of the Farm Administrators group. They constitute of all local administrators present in the physical server. You can remove this group if you already have added separate service account preferrably (Domain user based Service account) and Domain user based "farm administrators". Else, you will land up in access denied error on accessing Central Admin site.