Where are the SharePoint assemblies?
In SharePoint 2013, the SharePoint assemblies are located in the \Windows\Microsoft.NET\assembly\GAC_MSIL\ directory:
All other SharePoint files are located in what we call the 15 hive: \Program Files\Common Files\Microsoft Shared\Web Server Extensions\15.
When inspecting IIS, you will notice a change in the virtual directories: the _layouts folder still points to the 14 hive (containing the SharePoint 2010 files), while there is an additional sub folder that points to the 15 hive (containing the SharePoint 2013 files).
When navigating through the file system, you can see that there is still a 14 hive, containing folders like TEMPLATE, TEMPLATE\CONTROLTEMPLATES and TEMPLATE\LAYOUTS. When you edit one of the controls in the CONTROLTEMPLATES folder, you will notice that they still reference the old SharePoint v14 assemblies. If you deploy your SharePoint 2010 solutions and features to your SharePoint 2013 farm, it is in this 14 hive that you will find your files.
But there is no ISAPI folder anymore in the 14 hive, nor are the assemblies in the Global Assembly Cache. Calls to the old assemblies are redirected to the SharePoint v15 assemblies. This redirection is defined in special policy files located in the \Windows\Microsoft.NET\assembly\GAC_MSIL\ directory: