What is a YouTube niche?
A niche is not just a topic, it's an economic market of attention. Simply, a niche is the specific audience, specific style and specific format your channel is built around.
"History" is not technically a niche, but we use these macro terms when STARTING our research, of course. "World War II Military Strategy" is a more specific niche. "Middle Ages Politics" is also History but these are VASTLY different audiences.
The mistake most people make
The mistake most people make is picking a topic that interests them and calling it a niche. The result is a channel that has no consistent viewer, no consistent packaging, and no clear reason for the algorithm to push it to find an audience. A real niche has an obvious answer to: who is the returning viewer, and what do they get every time?
How to test one
Your goal here isn't to lock in your niche RIGHT NOW, but to test three potential niches against real data (authority channels and outlier channels) until one starts looking like you might just have a powerful opportunity.
The goal isn't to move blindly into a niche with a lower barrier of entry, it's to collect data to find a gap that is not going to be swallowed up by 1,000 slop channels. That's how you build a lasting channel.