jfc I'm not interested in a semantic conversation, but having firsthand knowledge of something means you heard it from the source as well as experiencing it yourself. You have now have second hand knowledge.
When you get information from someone who saw something happen or heard something said, that's firsthand evidence. And anything you see or hear yourself is also firsthand. Your summer job might be a firsthand look at what farming is really like. Interviewing a witness to a crime gives a police officer firsthand testimony about what exactly happened. This word, coined around 1690, comes from the idea that the maker of something is its first hand.
Definitions of firsthand:
adjective
received directly from a source
“firsthand information”
Synonyms:
primary
of first rank or importance or value; direct and immediate rather than secondary
adverb
from the original source; directly
“I heard this story firsthand”