Weird line shows up on mesh model for a gun, and on Blender Guru's tutorial a weird cube like shape appears with sub surf. Why?

Question

FOR THE ANVIL - It's really frustrating because i see people with the exact same problems as me but i feel like I've tried everything. Recalculated normals. Merge by distance. Getting rid of ngons, and just none of it seems to work! I bet the problem is something simple, but I'm new to blender so I don't know what to do. The cube also shows up on only one side, which is odd.

FOR THE GUN - I bet it's something to do with the way I modeled it, but in object mode there's a weird black line streaking across the model where certain edges are, but I just can't fix it. This brings up another question I have though - are ngons and tris really that bad? I was under the impression it's awful to have ngons so while modeling this I was making sure there were none, but it just led to more problems. Knife cutting led to more ngons, more ngons led to more unnecessary loop cuts, etc. So, if you have an ngon or tri should you go through the effort of removing it? If so, what's the best way to go about it without accidentally creating even more? Anyways, I hope that was clear enough for someone to answer because I felt like I was spewing out sentences.

https://drive.google.com/file/d/1OR4pz9iPGWh0PebqW5F3Lp4onq8lIZhF/view?usp=sharing

https://drive.google.com/file/d/1KEYJchV3R128Dj9B8BFQJH5Qm7bcTjL9/view?usp=sharing

Links to both of the blend files I'm talking about, hope this works since I don't know how to upload blend files here.

Answer 1

For the anvil: I have no time to fix it for you right now, but when I activate the Overlay > Face Orientation it shows that a lot of face normals are pointing inwards (shown in red) instead of outwards (shown in blue) as they should be. Also, looking at the top of the anvil there are definitely faces overlapping because there are areas where red and blue are mixed. Also notice on the side, faces are not only flipped but also overlapping. You can see it especially where the red and blue colors in these areas are more saturated than in other areas. It looks like you have a lot of doubled geometry.

The problem with Merge etc. is that most of the model is double faces using the same vertices - I don't actually know how to create this, but what it means is if you click a quad face and delete it, there is another quad face in the same place. Merging vertices doesn't help because both faces are built of the same four vertices.

Even selecting Interior Faces doesn't select them all correctly, I don't know how to get rid of them or if there is any function in Blender to select them automatically. So if nobody comes up with a better answer this is all to be done manually, I'm afraid. Clicking a face to select it, hitting X to delete it. Maybe you can select more at once with Box Select or Circle Select in Solid View, so that only the top faces are selected. But if you delete some and there are no more faces beneath them left, it might be harder to fill the holes if there are corner vertices missing afterwards.

Once you have all double faces deleted, you can select all with A, then Shift+N > Recalculate Normals. Now they should all be shown in blue.

Answer 2

For the anvil, you have faces overlapping, and Merge by Distance doesn't seem to fix it. A quick way to fix it is to choose Face Select mode, select everything in the 3D view with the box or lasso without the X-Ray enabled so that it doesn't select the inner faces, invert the selection, delete, then don't forget to recalculate the normals.

For the gun, there's a line because the top face is separate from the bottom faces, if you move one of the vertices from the bottom faces you'll see that they are not part of the top face:

It's not a problem to have ngons or tris as long as it doesn't create artefacts