What's the difference between a "direct drive" starter and a "gear reduction" unit? I've rebuilt a bunch of Mopar starters over the years but never a "direct drive" unit. However, in all the one's I did I don't recall any sort of gear reduction in them. The solenoid drove out the gear which slid on the armature shaft.

The gear is the reduction. Direct drive do not have that. Look at a service manual.

The Bendix is the clutched gear that engages the flywheel.
On a direct drive starter the bendix in on the armature shaft. The solenoid slides it along the armature shaft until it engages the flywheel.
On a gear reduction starter the armature turns a gear that turns the shaft the bendix is on.

And that's why they sound funny.

Anybody else notice that, in a lot of movies, the Mopar gear reduction starter sound is used on a variety of non-Mopar cars?

I notice the opposite, when it's a Mopar that sounds like a Ford or GM when it starts. One example is Adam 12. The Belvedere squad cars only occasionally sounded like they should.

i always listen, and i'm always amused when a brand "x" sounds like a mopar, and when a mopar sounds like a brand "x".

And you hear tires screeching on dirt!

They do all the time if the dirt is right, especially dirt oval tracks.

i used to laugh at that as well, until i actually heard that and saw "black marks" on the dirt !
if the conditions are right, tires can actually smoke on dirt surfaces.