Apologies for so many questions in a row! I'm kinda new and really want to learn.
My question is, why are we UV Unwrapping without applying the mirror modifiers for the main body and the treads? Does the mirror modifier also mirror the UV maps?
Questions are a great way to make sure you're learning! Ask away :)
The mirror modifier does mirror the UV maps, so for these areas you'll be using half the texture space for the same amount of detail. The obvious downside is that it could look obviously mirrored, so for other areas like the top of the tank you will apply the mirror modifier (if you used one and haven't already). You can see the result at about 10 minutes in.