I am fairly new to this and have a question as I always get distortions as soon as I try to extrude any verts of this cylindrical shaped part of the mesh or when I try to connect Verts of this part with other verts from parts of the rest of the model.
The Subdivision Surface Modifyer is causing the distortions. As soon as I switch it off, it's gone. I applied All, flipped the normals (and flipped them back afterwards as it had no effect), Merged by Distance, tried adding a crease around the edge loop, added additional edge loops etc.
Has anyone any idea what is causing these troubles?
I added all the faces around the Edge Loop at the back and the distortions on the outside of the cylinder are finally gone indeed, but there is a white ring clearly visible now inside the cylinder.