A niche blogging or site is one that targets a very particular group of people.
Flipkart, for example, is not a niche site because it sells a wide range of items. Same as Amazon, a news website that is not niche blogging site.
But if you see some website like Spotblogging.com - that is based on blogging tips and tricks. they do not post other articles.
Another example is backlinko.com which works on backlink tutorial. They are the blogging tutorial-focused website that also serves as an expert on the subject.
There is a significant distinction between authority and specialized websites. Websites with a lot of material that teaches you about a specific topic are known as authority sites.
A niche blogging or website is one that teaches you anything about a certain subject.
Weight loss, for example, is a large authority issue rather than a specialty. “How to lose abdominal fat after pregnancy” might be a specialty.