Multiple network question - sharing drives / other

[inflames988]

Currently my laptop and desktop are on two separate networks. One is a wired network where a switch connects the two, and they are also on my home wireless for internet and sharing with other computers. I have network folders shared out on almost every computer in the house. My laptop uses a hard drive on the desktop for backup purposes. Now im wondering, when i save things to the network drive (which puts it in a folder on my desktops external hdd) from the laptop, which network is it using? The wireless, or the wired? If one is disconnected, it uses the other. This might seem hypothetical but i was just wondering. On to question #2:

Ill be moving into my college dorm in two weeks. I want to use the switch to keep up my ability to share files between the computers, but i DEFINITELY dont want anyone to be able to access the shared folders on them besides me. I dont know the specifics of the network there, but my laptop will be connected wirelessly to the internet and with a wired connection to the switch, and my desktop will be connected using two wired connections, one to the internet and one to the switch (it has 2 ethernet ports). Is there a way to ensure that ONLY this laptop can access the shared drive on the desktop, and vice versa?