How I can get rid off this strange line that appears on my model? and how I can connect better the cut in the middle with the rest of the mesh?
Try to make your topology more even, and use supporting edge loops around all the sharp edges.
Welcome!
The circular shape in the center should be connected all around to vertices on the outer section.
The sharp lines should be marked sharp and auto-smooth should be active so those sharp edges are properly hardened.
If you are going to bevel the corners, the bevel should also apply to the edge all the way, or it will cause a weird shape that will shade weirdly.
These changes might require doing the various features in a different order or with different methods than what you are doing.
Here is an exemple. I am not sure I fully understand the shape of the object, as there is only 1 angle in the images, but I tried to roughly replicate it.
topology_test.blend (843.2 KB)
In 3D software, everything is rendered as triangles. This means that a face with multiple sides is internally split into triangles so it can be rendered. If the face is curved, this splitting process will often result in a jagged and crooked result.
If you are modeling a complex transition between a curve and a straight edge, you will want to split the polygons yourself. If you just use a single face with many sides, you lose control as Blender will have to split and triangulate that face itself, giving you strange results.
it’s fixed. Thanks for the explanation : )