I spent many years in studios but am completely new to DAW and don’t understand the way of mixing down cubase uses. In the manual under mixdown it seems to describe exporting to a file rather than mixing so there is no control on track volumes while mixing.
How can I use the software as if I had a mixing desk in front of me and controlled the slides as if I were mixing down to a 2 track machine, or in this case, a file) ?
A mixdown is a mixdown.
Press f3 for the mixer.
You need to record fader automation (if you want to ride the faders) before the mixdown.
Once you are happy, do a mixdown.
Or route to group and record mixdown as stereo. Make sure your “mastering plug” are on one if those group and not stereo out (otherwise not recorded). Use hour cadets live while it is recording.
Old school terminology and new school. Before DAW there was a mixer and a master recorder with tape, the record button was pushed and then the hands would do the automation. This was called a mixdown. In the DAW world it easily could have been named “rendering”
Kewl:)
I used to do that and record to DAT… Does that make me “medium school”?
Got anymore old school stories?
Yep, you’re in the middle 
Have a look at the section about automation.
Enable automation writing, either on a track by track basis or globally across the mixer. You can then compile your mix by fader (and spins to sends, eq sweeps, etc) automation which can be saved and tweeked. You can do this in one pass or many.
Having some sort of fader equipped control surface can help a lot with the process of compiling the mix automation.
When you have the mix with it’s fader rides etc to your satisfaction, then do the “Audio Mixdown” which is as you and others have said is basically a file render operation.