System design for clients in large scale applications

Recently I got interested in large scale applications like Amazon, Twitter etc. and I find whole lot of information on how manage the server side of things but that makes me thinking about the client as well. I am interested in finding out how clients of these massive applications differs from client of small scale applications? What are the design considerations? Are there any specific architecture patterns one can look into?

Also a additional specific question, while reading about the type-ahead system, I came across a question -"What is happening on client side". I am clueless to this except that client might have created a websocket connection to send the data as its being typed and server might be sending back the data continuously and for reason, I am still not able to imagine anything special happening on the client side.