user46022: Or where can i find information of default bootloaders present on each Arduino board. All I could look up was it could be one of these bootloaders on the atmega chip :-
.Atmega
.Atmega8
.bt
.caterina
.caterina-Arduino_Robot
.caterina-LilyPadUSB
.gemma
.lilypad
.optiboot
.stk500v2
This was the list present in the folder ~\Arduino\hardware\arduino\avr\bootloaders.

user47766: Optiboot Bootloader for Arduino and Atmel AVR
https://github.com/Optiboot/optiboot/wiki
Take a look at this information. 

user30198: It is defined in the file "boards.txt" (arduino-1.8.7\hardware\arduino\avr\boards.txt).
For example the Arduino Uno is called "Arduino/Genuino Uno" and it has the bootloader: uno.bootloader.file=optiboot/optiboot_atmega328.hex
You can find that hex file in the folder arduino-1.8.7\hardware\arduino\avr\bootloaders\optiboot.
The official Arduino boards are shipped with the newest bootloaders.
Sometimes the bootloaders are improved or changed. That is mentioned in the Release Notes.
For example the latest change to a bootloader is that the Arduino Nano has now the same optiboot bootloader as the Arduino Uno.  
When buying a Arduino clone board, anything is possible.
Some companies make there own Arduino-compatible boards with their own bootloaders.

46022: <p>Or where can i find information of default bootloaders present on each Arduino board. All I could look up was it could be one of these bootloaders on the atmega chip :-</p>

<p>.Atmega
.Atmega8
.bt
.caterina
.caterina-Arduino_Robot
.caterina-LilyPadUSB
.gemma
.lilypad
.optiboot
.stk500v2</p>

<p>This was the list present in the folder ~\Arduino\hardware\arduino\avr\bootloaders.</p>
 47766: <p>Optiboot Bootloader for Arduino and Atmel AVR</p>

<p><a href="https://github.com/Optiboot/optiboot/wiki" rel="nofollow noreferrer">https://github.com/Optiboot/optiboot/wiki</a></p>

<p>Take a look at this information. </p>
 30198: <p>It is defined in the file "boards.txt" (<code>arduino-1.8.7\hardware\arduino\avr\boards.txt</code>).</p>

<p>For example the Arduino Uno is called "Arduino/Genuino Uno" and it has the bootloader: <code>uno.bootloader.file=optiboot/optiboot_atmega328.hex</code></p>

<p>You can find that hex file in the folder <code>arduino-1.8.7\hardware\arduino\avr\bootloaders\optiboot</code>.</p>

<p>The official Arduino boards are shipped with the newest bootloaders.<br>
Sometimes the bootloaders are improved or changed. That is mentioned in the Release Notes.<br>
For example the latest change to a bootloader is that the Arduino Nano has now the same optiboot bootloader as the Arduino Uno.  </p>

<p>When buying a Arduino clone board, anything is possible.</p>

<p>Some companies make there own Arduino-compatible boards with their own bootloaders.</p>
