I assume the joke is (usually undocumented) people look for work in the parking lots of Home Depots, hoping to be hired for day labor, and the same may happen with the nerd glut in our culture and Best Buys.

I've never witnessed this as I live in a smaller city in Northern MN but it's a cliche here in the States.

Home Depot is a home improvement warehouse. "Hardware store" is underselling it a bit as they sell lumber, building supplies, appliances, flooring, roofing, lawn and garden supplies, etc. You'd be better off buying from a wholesale lumber yard or building supply, but Home Depot has a contractor supply program. You can buy everything you need to build a house out of a Home Depot. It's a direct competitor to Lowe's; if you watch videos about American woodworking you may hear them obliquely refer to "orange and blue big box stores." Home Depot's logo color is orange, Lowe's is dark blue. Home Depot also has a theme song that slaps harder than it has any right to. And "big box" is apt; you can measure the average floor space in a Home Depot or Lowe's in acres. Home Depots tend to stand alone with their own parking lots.

It's something of a stereotype for day laborers especially Mexican immigrants to hang out in the parking lot of a Home Depot waiting to be hired for work. Need a building roofed? Go to the Home Depot parking lot and hire a half dozen Mexicans to get it done cheap, fast, and probably well, so goes the stereotype. I associate this image with the Southwest, I've never personally seen this here on the Eastern Seaboard.

Best Buy is a big box electronics retailer, they sell televisions, computers, cell phones, game consoles and video games, etc. They have a reputation for not having many customers because most people shop online now; in fact there was a joke going around that Best Buy was Amazon's showroom. People would go into Best Buy to see the product in person, leave the store and order it from Amazon cheaper. Best Buy is also known for their in-house tech support staff called the Geek Squad. Best Buy sort of sells PC parts, compared to something like MicroCenter they have a pathetic inventory of ATX stuff but I have bought the occasional SSD from there, they also sell garbage tier laptops and tablets. Best Buys often anchor strip malls and will have other stores attached to them, sharing one long parking lot.

OP is trying to conjure an image of a bunch of IT guys and developers milling around in a strip mall parking lot waiting around for someone to hire them to configure a server or something as if they're undocumented immigrants looking for work paid under the table.

I didn't get it either and am American.

Can someone explain the joke

Day Laborers Home Depot Parking Lot

I have not seen this myself, but I have relatives that own several rental property and they often see people in Home Depot parking lots just standing around waiting for work. They'd then talk to them and offer them to work on their properties. I think the reason is that these are likely either undocumented immigrants or new immigrants that cant find a job, so they just stand around hoping someone will offer them work. They are probably cheaper than a US citizen worker doing the same thing. Although, the quality of work will vary, and the quality of their work is... questionable. (I mean, they aren't gonna have licenses or anything)

I'm not sure about Home Depot but Walmart is known for letting people camp in their cars on the parking lot where as many other stores don't so I assume this is a joke about IT people sleeping in their cars on the Best Buy parking lot.