I'm looking for an easy song producing app, where you can insert keys into a piano, and the notes appear on sheet music on a computer.

Does anyone know the name of the app? I'm pretty sure I saw it somewhere, I just forgot what it's called.

I know this is very vague and I apologize for that. I'm a beginner in music theory and I want to see what it's like to produce an a capella song on my own.

Also, Android preferred if it's an app.

I've never heard of any music notation apps, since I'm not really a mobile device kind of person, but if you're willing to work on a computer then I would recommend Finale. I've been using it for nine years now, and it's really quick and convenient once you get the hang of it and memorize the keyboard shortcuts. I've upgraded to the full version of the software, since I was working on larger scores, but if you want to write A capella music then Finale Notepad (the free version of their software) should do the job just fine. You can write up to eight staves, it works with input from a MIDI keyboard, and you can add lyrics. I've never used Finale's MIDI input system myself, but it doesn't seem like it would be too difficult.

Here's a link to the Finale Notepad page, if you want to download it. https://www.finalemusic.com/products/finale-notepad/

If you're looking for other suggestions, I briefly used Noteflight once, which has the advantage of saving your scores online rather than to a computer. Other people use Sibelius, but I don't have any experience with that program so I can't give you my opinion on it.

Best of luck with your compositions, and remember to check your counterpoint! Parallel octaves kill brain cells!