There are many issues in the post, but I will cite a few.

I decided to begin a series about some Youtube downloaders I’ve used.

I ‘’decided to begin’’ is extraneous. You could say: ‘’I began a series about some Youtube downloaders I’ve used.’’

The style used on the interface is a little bit different.

A little bit is redundant. A little or a bit, choose one.

This is why I assume it to be performing two functions.

Readability could be improved with: ‘’So I assume it to be performing two functions.’’

This feature is the main reason why this app is important.

Another tautology. Reason and why.

I said NewPipe’s UI is great. Especially for the fact that it had dark themes.

Here, the second sentence is a fragment with excessive words. That could read as: I said NewPipe’s UI is great, especially because it had dark themes.