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.
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.