The first camera I ever held, and pretended to take photos with, was probably my Dad's Kodak Duaflex III.
I still have it. I love it and the memories that go with it. I haven't put film in it in over 35 years.
TTV photography is using the viewfinder on similar old large viewfinder cameras to take your photo through (using the digital).
One tutorial on what you need to do TTV is this one: Russ Morris
I have made 2 tubes, one long one to use with my 105mm macro and one shorter one that works for closeups with the 50mm +20mm closeup ring