1) Trash on the ground must always be removed by the person to whom the ground belongs. Regardless of your situation. If the property owner knows who dumped the trash there, he can ask the perpetrator to remove it. If the perpetrator does not do so, it is initially your trash. You can then take it away / dispose of it (do not dump it at the perpetrator’s place). Afterwards, sue him for the disposal costs.
That’s exactly why I’m asking now – it’s not mine yet and I actually have nothing to do with it. If I buy the property with construction debris, it suddenly becomes my problem. I will call the building authority afterwards and make them aware that the construction debris must of course be removed before the purchase – how they handle that is their business!
2) even if there is a post in the ground. A post is initially just an orientation. A property is marked with boundary stones at all corners if it is properly surveyed. That is the border. A stick in the ground says nothing at first. Maybe it was knocked over once, its maker put it back where he assumed it should be driven into the ground again… Maybe even 2 meters into his property, so that visually it first looks like he is building on “your” property.
No idea where the stick comes from now, but I am 100% sure that it’s wrong. I will sketch it out for you briefly after work today around noon, then you’ll know what I mean.