Hello, if I used unreal engine to make my own game ,am I free then to publish it on app stores like google play or sell my game to big companies for example,please I would like to know more about these concepts.
Yes, but not for free. Epic will take a 5% cut.
For legal information, visit: Frequently Asked Questions - Unreal Engine and Release and Royalty Tracking Guidelines - Unreal Engine where you can find more information and e-mail addresses you can ask for more information.
is correct about the way of going about publishing and letting us know. One thing to note however is the 5% royalty is only owed after the first $3,000 per business quarter. So you can earn up to $3,000 without paying royalty fees on it, then the money you make past that will require a royalty be paid from that.
Thanks so much ,but I have one more question please,so if I publish my game in app stores then should it be free or charged ?am I free to decide?should I pay 5% royalty if my app is free?and about paying royalty should I send you them by myself or there is a certain system that does that?
You do not have to pay if your app does not make revenue.
Release and Royalty Tracking Guidelines - Unreal Engine Here is all the information you need. A step by step guideline.
Once you send the information. Epic will provide payment instructions.
Most probable that you have to send it them by yourself. A system that detects your revenue transered to your bank account made by your ue4 game is not present. But you are due to have a legit administration. (Like any other organisation)
Hi. I also have this sort of question but probably not worth to make a new question on top of it. If my game uses other assets, from the marketplace (like the free Epic games materials for example), can I publish my game? And if I can, what do I need to do to publish it?
Thanks and have a good day! 
Yes, you are allowed to release a game to the public that uses Marketplace assets under the terms of the EULA.
When you are ready to publish the game, please follow the steps outlined on https://www.unrealengine.com/release.