In the Dungeons & Dragons game which part of the material is based on actual facts in history, mechanics or science and not all about fantasy?
I will base some things on fact like physics and gravity but some materials can be stone wood iron steel or a mixture of other fantasy stuff.
Generally I feel they use the same names of items found from the middle ages including whatever legends and lore they had but largely it is all fantasy specially anything related to magic or the supernatural.
Various components in Dungeons & Dragons are based on reality, mostly mechanical parts and elemental effects. Some historical components are incorporated into the game system, but much of the Rest of the game is fantastical. When you have a system that has magic involved, no physical law need apply unilaterally.
A pause from D&D Game Material Based On Facts to share caption from old times:
Today is: 22nd July (GMT), in history on the 22nd of July, 1992 AD the following event happened:
Soyuz TM-15 launches