it is simply webapp made in cordova ,that is converted to apk app, that shows iframe of the website, in this case it shows
<iframe src="https://rule34.xxx/"></iframe>
the code of the whole app is here: https://github.com/sadedade/rule34.xxx-Android-Client/blob/APK/www/index.html
if you dont trust my build you can make it your self for fun, its not that complicated. And it can seem pretty sus that the file is 1MB, but that is just how small the app is.