Your question is not clear to me. how can we make APKs from the code? or how do I make APK from an existing application(installed from play store)? If I consider the first one then there are a ton of frameworks available to get your job done. Select whatever framework you wish to use. Then follow it's documentation to convert your code into an APK. For the second case use any back-up application to make the APK for an installed application. I won't suggest people to search for an APK file on the internet . Backing up an application is always safer than downloading it from the internet. Thanks.
I just know C language and I want to make APK so how should I start?
Short answer is you use the Android native development kit. Android NDK | Android Developers s . You can call the Dalvik Java classes though an interfae that is part of the NDK. If you know C and SDL the transition isn't that hard. I suggest you learn SDL. As a rapid protor you can use X11Basic. Yeah I know it's BASIC but if you learned on the old Atari ST with GFA Basic then you'll be making apps straight off X11-Basic Homepage . You'll have to tackle packaging and manifest using the basic tools but if you know make well enough this isn't difficult either.
How do I make mod APK for Android games?
Well if you are intended to make mod of games then I am sure you have rooted phone. If not then root you phone. Well I think you want to make apk of games that has unlimited stuff like coins and gems. OK then download lucky patcher and install it. Now the actual process began. Open luck patcher and click on the game that u want to hack. Click open menu of patches and click on support patch on inapp .. Lvl emulation. It will take time and then launch the game and try to do a purchase of extra gems and coins and you will notice that currency sign are removed from them like $25 changes into 25. Then you are done click on it and in offline games you have 99% of getting successful but sadly in online games only 1%. Well above method creates mod on you device only. Now to create mod click on create modified apk in option in lucky patcher of your game. Good now u can create it and share it on non rooted phone too Happy experience)