"SAM" folder is missing in "hardware" folder

I'm currently looking for bootloader on atmega16u2 and SAM3X8E for Due and source codes for them and stuff, but I cannot find it on my computer at all. The folder "SAM" is missing. Why is it not included to standalone Arduino IDE? Could anybody please point me where to get these files?

On Linux systems, the sam folder is in a hidden directory. Be sure to use Boards Manager to install Arduino SAM Boards support.

~/.arduino15/packages/arduino/hardware/sam/1.6.6/

bbx10node: On Linux systems, the sam folder is in a hidden directory. Be sure to use Boards Manager to install Arduino SAM Boards support.

~/.arduino15/packages/arduino/hardware/sam/1.6.6/

Thank you. I found sam diretory on linux, but still cannot find it on Windows. Also sam folder on linux is lacking SAM3X8E's bootloader. I would like to learn it. The internet seems lacking any information about this bootloader. Could anybody help?

In a Windows command window try the following.

cd %userprofile%
dir/s arduino15

This returns 2 folders on my system. I suspect the version of the IDE determines which folder is used.

AppData/Roaming/Arduino15
AppData/Local/Arduino15

For source code, I suggest searching in https://github.com/arduino/Arduino/

bbx10node: In a Windows command window try the following.

cd %userprofile%
dir/s arduino15

This returns 2 folders on my system. I suspect the version of the IDE determines which folder is used.

AppData/Roaming/Arduino15
AppData/Local/Arduino15

For source code, I suggest searching in https://github.com/arduino/Arduino/

Thank you.

As for the bootloader for SAM3X8E and it's source code. Is it unavailable for public? Could anybody confirm this?