How do Te's form their own opinions?

Yeah, that's right. I've been studying MBTI for almost a year and I still don't understand some things about Te. It's a function that forms its opinion based on something external, but I've seen many dominant Te's creating their own understanding against what most people think, how do they arrive at that? Te is a function that I often envy for its productivity, but the way people describe it seems to make it seem like a function without its own identity, when in real life I've seen people who are clearly Te but who seem to have a lot of crazy beliefs. So how does Te have self-created opinions if a dominant Te in theory tends to take almost everything from the external, from what already exists and not question them very deeply?