Which is the best app for someone who doesn't know anything about investing?

There are 2 aspects of investing

a) Which stocks

b) Executing the transaction

There is of course a number of related questions to the first aspect eg how much to buy, when to buy, how to mitigate risk.

No app is able to help you answer these questions properly. To do so, you need to learn how to invest.

Once you have learned how to invest, then comes the execution part. This is when you look for the appropriate app.

There are several investing styles eg technical, fundamental, quant, etc Most apps are good for one particular style.

Asking which app when you don’t know anything about investing is like asking which calculator to buy to enable you to be a good mathematician. There is no calculator that can teach you how to translate a problem into a mathematical equation. Of course, once you have this, then a calculator is useful.

So don't put the cart before the horse. Learn how to invest first and then looking for the appropriate app is the simplest.