What bootloader is present by default on Arduino Uno?
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.