Picking a Youtube topic
Picking a niche will affect your success on youtube because youtube delivers your content to an initial group of subscribers, and the people that click on the video and the watch time affect how far and wide youtube will promote it. And the opposite is true.
Switching topics for established channels might hurt in terms of views, because you built your audience around a niche, and that audience might not be interested in your new content, which will limit your video's reach.
How to pick your niche on youtube
Ask yourself:
-what do you enjoy talking about, what's your passion? list your hobbies, interests...
-what do you have deep experience doing? what would like to learn and take your audience on a journey of learning with you?
-what is the most profitable niche? combine what you are passionate about with what is in high demand!
Once you choose the topic, make sure you narrow it down to a niche.
Be different, from the things you wear, your tone, your way of delivering info, your style...
Extra tip: Find five channels that serve your niche and watch some of their videos, to see which content works best, and make sure you see where you can be different, in order to shine.