Booleans!

Union.

Union is a useful tool that allows for Models vertices to be combined seamlessly and without clipping. For this, the torus shape what was clipping through the mesh, Both are now the same object, with their vertices merged, Any edges or verticles that were on the inside are gone, leaving only a clean outer shell.

Difference

Difference is a rather interesting tool, that allows for you to make dents, holes or even marking in meshes, an example would be a face print in a rock or mud. Any intersecting section of the models are deleted, but this depends on the order of which the models are selected.

For example, if I chose the head of the AT AT first, it would not be deleted and it's core vertices would remain.

BUT, if I selected the torus first, the torus would remain (Like in the picture below) While the head of the AT AT is deleted.

Intersection.

Intersection is like the exact opposite of intersection, where any faces and vertices that ae clashing, are kept while the non clashing vertices are removed. This of it like taking a container over jelly, taking out a section of said jelly. You have what the container managed to overlap with.