Is a niche store better than a general one?

For anybody without a large budget, yes and by a wide margin. A narrow store can be found, can be explained in one sentence, and can be genuinely better at something.

What decides it

The fear is that the market is too small, and it is almost always the wrong fear. A store that owns a small category can widen later from a position of trust; a broad store with no reputation has nothing to widen from.

The part that gets skipped

A market that already spends money is easier than a market you have to convince exists. Competition is evidence, not a warning.

Test whether people will pay before building anything. A hundred people saying they like it is worth less than one paying.

An idea is worth almost nothing on its own. What is scarce is somebody who will keep going after the first version does not work, which is every first version.

Where an app fits

If the idea is a shop, the sequence matters: sell first, prove people come back, then build the app. An app before the customers is a cost with nothing to do.