Isn't the AllItems.aspx instance page  for every list/library based upon
the ...\12\Pages\viewpage.aspx?

If you are talking about making changes to only one  allitems.aspx,
then you can use SharePoint designer to locate the file for the
particular list/document library.

Before you make any changes to the given file, make sure you make a
copy, then work on the copy, and then associate the copy with the
list/doc library.

Infact, i have been able to dig down, and find  the exact master
page, and put some necessary tags.

But the problem is, that the area rendered by the following control,
in the Master page, is too general.

<asp:ContentPlaceHolder id="PlaceHolderMain" runat="server">
</asp:ContentPlaceHolder>


How can i find the exact control behind this, so i can make changes
in more details? I tried to search through ID, etc. but it seems it
is a web control ?

All the content  rendered by the ContentPlaceHolder who's ID is
PlaceHolderMain is physically located on whatever page  has been
requested. ON that content page, for example default.aspx there will be
an <asp:content ContentPlaceHolderId="PlaceHolderMain" ... tag.
Everything inside that tag will be inserted into the tag on the
Masterpage when the page is rendered. The tag on the masterpage
identifies positioning and the tag in the content file encloses the
content. That's how MasterPages work.