The first illustration shows a typical ceramic heating element.
These are manufactured using ceramic “bobbins” or segments which, when assembled can form a core of varying length.
The segments are shaped to allow a resistance wire to pass through and be terminated at one end of the element, called the terminal block. This is normally made of a different and stronger material than the segments. Resistance is calculated to produce a specified heat output with respect to the fluid being heated.
The primary advantage of the rod type is the capacity to bend into a shape suitable for the process, as can be seen from older style cooking hob appliances using circular shapes.