Or in other words ... . Does the DBMS somehow circumvent the OS Cache manager and acccess the memory directly OR it lets the OS do its job, behave as a normal process and use the concept of virtual memory?
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Virtual Memory is an indirection mechanism that map virtual addresses to physical addresses and the translation occurs in hardware layer through PTEs. A process can refer and address only virtual memory. Only privileged execution (kernel) can address physical memory. I recommend you read a book on the subject, the classic being Windows Internals by Russinovich/Solomon.
Your explanation is asking though a completely different question, whether OS 'Caches Manager' is circumvented. Virtual Memory is not a cache and there is never any sort of 'caching' in any discussion of a memory manager architecture (on any OS). It is not clear what 'cache manager' are you referring to. Does SQL Server do buffered IO (ie. uses OS file cache)? Generally no, see How It Works: Bob Dorr's SQL Server I/O Presentation.