App stores

I hope that this is the right forum for my Q. Interested in building a android app and one for the iPhone and am curious if they split the sales revenues evenly between them and app owners? 50/50% and why would I give them 50% of the profit what are they doing for me exactly? Do they own 50% of the app too? If so that means they can take it from the app store if its not doing high sales I guess?
Thank you for the replies.