For those who don't know what a Linked Server is, it is SQL Server equivalent of DB Links. A linked server configuration enables SQL Server to execute commands against OLE DB data sources on remote servers.
For running a query using Linked Server, you first need to create the linked Server by giving you remote server configurations. For more detailed info on creating it follow the link.
Once you have created the Linked Server, say with server name NVIDIA and database schema named demo, and now you want to query table PURCHASE_ORDER, then do the following.
SELECT * FROM [NVIDIA ].[demo].[dbo].[PURCHASE_ORDER]