These are more of industries and are very broad. You will compete with big players which is no-no and bad advice in my opinion.Something you are passionate about. for example. wealth is a niche. dating is a niche, health is a niche and the list is long.
Great job.By definition- denoting products, services, or interests that appeal to a small, specialized section of the population.
For example,
Classification of Telegram
Sector: Information Technology
↳ Industry Group: Software & Services
↳ Industry: Software
↳ Sub-Industry: Application Software
↳ Segment: Online Chat
↳ Type: Instant messaging
↳ Niche: Anonymous & Encrypted Instant Messenger
https://en.wikipedia.org/wiki/Global_Industry_Classification_Standard