A torus is defined by two parameters: the major radius, and the minor radius. The major radius (
t.x) is the radius of the big ring (in red in the diagram), and the minor radius (
t.y) is the radius of the circular cross-section. The
y here are just indices into the vector: they're unrelated to the x and y axes.
The left side of the diagram shows the xz plane, and the right side shows a cross-section with y going horizontally.
p.xz is the projection of the point onto the plane of the torus, so
length(p.xz)-t.x gives the distance from that projected point to the inside of the torus. (I've labelled it $r$ on the diagram.)
p.y is the distance from the point to the plane of the torus, so the length of the vector ($r$,
p.y) is the distance from the point to the inside of the torus. (Computing the length of a two-element vector is a handy way to use Pythagoras' theorem to find the hypotenuse of the triangle.)
Like the torus, the cone code uses the fact that the shape is symmetric about an axis: the z axis in this case. The cone is defined by a unit vector
c that's normal to the surface of the cone, and the apex of the cone is at the origin. Similar to the above,
length(p.xy) gets the distance from the point to the z axis, so that we can work in the plane where the cone is a triangle.
vec2(q,p.z) is the projection of
p into this plane. The dot product projects this vector onto the direction
c: how far the point is along this direction tells us how far it is from the surface of the triangle.