Dial-up. :P Forerunners linked to the Domain using various technological means - their armour, supplemented by help from their ancilla, allowed their minds to connect to it. Cryptums and Terminals were also used to connect to the Domain as well, the latter generally for the purpose of storing information. In Silentium, accessing the Domain is described as a series of hallways, corridors and caverns, the walls being imprinted with the records and memories that had been stored there. Bornstellar had a similar experience in Cryptum where he thinks he can identify things that look like massive structures which he describes as being beautiful but can't reconcile their image into any definite shape or form.