Oil drain plug loose?

Usually that means the threads in the oil pan are worn and the plug loosens up from the heat cycling. The common solution is to use a tap to create new threads in the pan that fits a slightly larger drain plug. There are self-tapping ones that will cut their own threads, you can use one of those and then replace it with one that doesn't cut new threads. The other option is to use a kit to install a threaded insert in the pan to replace the aluminum threads with steel. Both work. What I do with either is to pull the old drain plug, drain out the oil. Now do the repair and once done, dump the old oil back in. Now drain out the oil and that should also flush out any chips. With either method of repair you will want to coat the threads or tap with grease to catch chips as well. You can get both types of repairs at a parts store.

https://www.youtube.com/watch?v=o16UEEbbcSU

If the surface isn't flat, then someone either cross thread the drain plug, or it has already been damaged and not repaired properly. From the factory the same tool that threads the hole uses the tap as a center to face the pan flat around the drain hole. There are only two things that can wear the threads on the steel drain plug or the soft aluminum threads in the pan. Those are all there are. If it's crooked you have three choices, bore it out straight and use an oversize bolt, install a thread insert after drilling it out so it is straight or replace the oil pan with a new one. The first two are commonly done to save you from doing the third.

On an oil pan the threads are the only items that can wear. They are meant to last through the warranty and the required oil changes and as such are generally not really all that strong. On a steel pan they are not a part of the pan when it is built, instead they are an added piece of steel that gets spot welded to the inside of the pan like a nut. Just like most fasteners, repeated use will cause wear, the threads are the only item that will wear. What you could do is try an experiment, go get a copper crush washer to use between the pan and bolt. Now screw the plug in and gently apply torque/ Now remove the bolt and look at the contact area the bolt and pan left on the washer. Is it an even pattern all the way around? If not then somewhere along its life the drain plug was cross threaded, that will do more damage than 50 oil changes because of the way it crushes the threads. In the thin materials used it is also hard to correct unless you remove the damaged threads and cut new ones that are oriented at 90 degrees to the pans surface. The aluminum ones are easier to do that with though as they have longer thread runs. The steel pans can be difficult if you don't use a guide. For them there are kits that use an oversized tap but have what is essentially a large nut with the new threads in it, you screw the nut on the tap and then use the tap to thread the pan with the nut acting to keep the tap at 90 degrees to the surface.

They might look good and still be worn. When you put the plug in can you rock it around in the hole? If yes, they are worn. You say the plug keeps getting loose. If you are tightening it to the proper torque with a nylon or copper washer and it loosens up, the threads are worn. Once torqued it should hold the torque value until removed.

That looks like aluminum. If torqued properly it shouldn't loosen up either but I prefer copper, then nylon, then aluminum.