What are those weird things?
They disappear when I zoom in but reappear when I zoom out far enough…
It seems they are not real geometries because I can’t select them and they disappear when I zoom to them…
Really annoying. What should I do to them?
Can you share the model? Without doing so the best we can do is guess.
I’ve seen things somewhat like that due to a defective OpenGL driver. But there could be other issues, and as @TheOnlyAaron wrote, sharing your skp file would let us see whether we hit the same.
Most often things like this appear when your model is either very far (say, tens of kilometers or more) from the SketchUp model origin or has parts (a stray line is enough) that are far away.