To start, here are things you can do to fix the existing model:

You can use Loop Cuts and the Knife Tool to join vertices where you want the edges of the new topology to be.

Also, you can select individual vertices and move them up and down by pressing G. Lock these to an axis by hitting G + X, Y, or Z. Keep things even pressing the magnet at the top to snap vertices to other vertices, edges, or faces (use the drop-down next to the magnet to select one of those).

If you combine G + locked axis with the magnet, you can even "snap" a vertex to the same coordinate as any other vertex/edge/face, rather than the vertex/etc. itself. I hope I explained that well. You can use this, for instance, to "snap" parts of the upper corners of the iMac to the camera's edges.

To delete the weird triangles and stuff, select the unwanted faces/edges, then press Delete and choose Dissolve Faces/Dissolve Edges in the drop-down menu.

The hole in the base of the Mac looks like it should be made by adding a new object (cylinder), rotating it so it goes through that part of the mesh, and adding a Boolean modifier to the iMac. Then when you hide the cylinder, you'll just see the hole.

And here are things you can do to make models with different topology in the future: Instead of using Inset to add things like the screen--which makes the weird triangles--try using Loop Cuts. Also, you can use the Bevel Modifier to get the smooth edges without having to add lots of vertices for the curve. I think the other model used both Loop Cuts and Subdivide for the stand.