When I get in the car for the first time after it's been sitting for a while, it smells like dirty laundry. I can't figure out if it's coming from the upholstery or the A/C.

If ti's the A/C, it sounds like it would be a ***** to get the mold or whatever cleaned out, but I haven't tried anything like that before.

Does anyone have any experience with this stank?

Normal. Search this site for "dirty sock smell".

A very common issue amongst new cars and most Mercedes. Search around and you'll see lots of reasons, primarily due to sealing of the vents at shutdown while there is still condensation in the system. In some models Mercedes will do a free sanitizing. Once clean if you follow the proper protocol for shutdown, you won't have many future issues in my experience. If you decide to off brand, make sure whatever coil cleaner you use is not so strong where it etches the condenser. Ozone works but only kills the bacteria or mold and does not clean the fins. Once clean, ozone is more effective. The fist time I encounter this was in an e90 bmw more than 15 years ago and not much has changed.

Curious, what is the proper protocol??

MBs in (certain markets) have a thing called residual heating which allows you to turn on the fan to blow at the vents after you shut off the car and lock it.

OP, try using ozone but be careful it might damage the interior plastics and leather so use an amount suitable, don't breathe it in, it is not good, do it in a ventilated area, hopefully that clears the smell.

Ya the REST button. We should be using this? In what situations?
I used it a couple times in the winter when it was dead cold to keep the cabin warm as long as possible.

Every time you want the cabin to maintain temperature as long as possible after engine off, use it in the winter use it in the summer, winter it will blow remaining warm air from the engine, summer it will blow remaining a/c from the vents.

How does this help with removing wet sock smells?Something I haven’t encountered yet.

no it doesn't remove it, it prevents it as it clears the trapped water from turning to mold.

I went down a rabbit hole searching here for "dirty sock smell". Some interesting stuff.

Fortunately or unfortunately, this is the first Mercedes I've had where this was an issue.

I've never had a REST button, much less used one. I've never followed the proper procedure, except possibly accidentally.

I keep the climate control set to AUTO with A/C on 99.99% of the time.

I'll get this cleaned up, get a fresh filter in there, and see if it comes back and how long it takes if so... For science.

cleaning the condenser physically (chemicals, foam), then using ozone. Then after driving and before shutdown turning off ac and letting the fan blow (not on recirc) to dry the ducts. Basically you’re trying to not leave the system wet when it shuts down and closes all the flaps.

For now, the smell is gone though I can't point to a specific thing that resolved the issue.

It could just be that I've driven the car enough to blow out some crap that settled in there while the car wasn't being driven on the dealer's lot. It could be that I had the cabin filter replaced while the dealer was doing some other stuff (I didn't pay for it, but also didn't see the condition of the old filter). I also steam cleaned the carpets, floor mats, and alcantara, but I think the smell was mostly gone by that point.

I'm going to continue just leaving the A/C set to AUTO 24/7 as I've always done, and not follow "proper protocol" and see what happens.