How does this button work without any circuit closing device?
The gold plated interlocking concentric "rings" are two contacts of a switch.
When they are electrically connected by a resistance of less than say 10 k Ohms the circuit is activated.

The white "rubber" has a conductive ring on it which is close to but not touching the gold contacts - it is probably held clear by small protrusions in the rubber moudling. When the rubber is pressed from above it presses the ring onto both contacts and closes the circuit.

The conductive material is often made of a conductive ink or similar which may be loaded with carbon particles to make it conductive.