If the threads look ok, I’m guessing the same as OK above, probably the tech just forgot to tighten the drain plug. Maybe they got a phone call or a call of nature after they initially threaded it in, then upon their return to the job forgot to fully tighten it. It’s a bit of bad luck, b/c if they had simply forgot to install it, then they’d notice when all the oil they just poured in was puddled on the floor under the car. (Ask me how I know this … lol )
Just a fyi note regarding oil changes. Check the oil level before leaving the shop, then again when you get home. And the next morning, before starting the car. Then once a week or so. It probably wouldn’t have helped in your case, but still a good idea.
It’s not possible to know who’s to blame , that will have to be resolved by mechanics studying the oil pan and threads, maintenance records, etc. Preferably you want to have at least one of the mechanics doing the research and inspections who are working for you. One thing to check is whether the shop installed the proper drain plug washer, and whether it was a new one or not. On most cars a new oem washer should always be installed with every oil change. I expect the result of all this will be some type of compromise, where you’ll get a replacement engine, but not at the full price, at some discount. Maybe you’ll pay for the engine, and they’ll pay for the labor. something like that.