Is mostly market found dc dimmer a dc buck coverter ?
Anyone please generous enough to break down and explain its function circuit blocks

 

Do you have a link to the dimmer to which you referred?

 

The common dimmers I've used have a 12V input to drive 12V LED strings. The strings have resistors to control the current to the LEDs when driven with the 12V.

The dimming action happens when a transistor (typically a MOSFET) switches the LED string to ground at a duty cycle appropriate to the dimming level.

Most of the duty cycle control is done with a micro, often an ESP type so they can become "smart" dimmers.