How exactly does this work?

[HellFire2345]

I wasn't sure where to post this so I decided to put it here because it somewhat relates to GMod.

I still have Garry's Mod 9.0.4 installed (this isn't the point, so don't think "this is a gmod 9 thread" just yet). I'm aware that games are turning to steampipe, and some already have, including HL2DM and DOD:S. I'm also aware that GMod 13 has to mount steampipe games differently because it's a different filesystem or whatever it is.

But my question is, how does GMod 9 still mount HL2DM (and DOD:S?) fine even though they're on steampipe? Why does GMod 13's mounting have to be changed? I'm noticing that the DOD:S and HL2DM content overwrites some of the regular source engine content like the bricks in the gm_construct building turning a darker color and the explosion sound is different.

The whole steampipe thing is confusing, could someone clear it up?

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