EG is a client and does nothing. It's in most cases the SAS Workspace server which assigns the library.
The command to assign the library is a normal libname statement.
Pre-assigned is a definition made in SAS Metadata. When you connect to a SAS Workspace server (that's connecting to a SAS Server, a session get's created on this SAS Workspace server (using the SAS Object Spawner)) the SAS Metadata is read and the libnames for all pre-assigned libraries get executed. - Have a look at the properties of the library in SAS EG - there you can see the libname statement which was used.
Other ways of having pre-assigned libraries are:
- Library is assigned via SAS/Share
- Library statement is part of the SAS autoexec which gets executed while the SAS session on the SAS Workspace server gets created.