Why are there these blue marked faces and these different red corners? Did I something wrong?
[blue]Are you sure those are faces and it's not the reference image? It may also be faces that are part of another object.
[Red] I think those are inverted/ flipped normals. You can go to overlays and check face orientation. If they are red then they are flipped normals. Select all and press alt+n-> recalculate outside
Those faces in blue may the result of some modifier misbehaving? If you do have modifiers at this point, turn them all off and see if it goes away. Then you know it's a geometry issue coupled with a modifier and we have a start at the mystery. If not then we have to keep trying to figure it out. Post more screenshots and give us more context.
It was actually because of the bevel modifier. How do ich fix it? Here some screenshots.
I must be something to do with your geometry. But as we always say, you will spend so much more time trying to fix it and just puzzled to what's wrong and wont even understand best the solution, and you'll get much more frustrated, so it is much better to start over. Alarm bells will ring in your head, as if it was a defeat, but it's all the contrary, the refresh works wonders. When you start from scratch, you will absorb all the information better the second time around, since you've already made the mistakes, the knowledge will solidify in your mind and you'll get to where you left much faster and it's an overall win.
