Mathieu Mouy, yes the home button will sink a bit even with the home button gasket because the iPhone 6 uses a real button and needs to click down. I personally have an iPhone 6 and that happens to me too, but it is perfectly normal. It might click down deeper like mine possibly due to the gasket not being on enough. It should be fine nevertheless. Enjoy your iPhone! -Alex Z.
Sheldon, I would first try to plug the phone into an outlet and let it charge for a while and see if it turns on. If it does, then you are good but check the battery health in settings to see if you need to change the battery. If it’s 85% or lower, you should probably change it. If it doesn’t turn on, you probably want to change the battery and it should come back to life. If that doesn’t work after you changed the battery, it could be that the logic board was fried or the ribbon cables are not fully connected. You can follow this guide here to change the battery: iPhone 6s Akku tauschen You can also buy a fix kit for this here for the 6s: iPhone 6s Battery Have fun fixing! -Alex Z.
Alex C, you probably have a dead battery in your brother’s iPad. What I recommend you do is get a battery replacement form apple or a local third party repair shop. You can do by yourself using this guide: iPad 6 Wi-Fi Akku austauschen, but it is extremely difficult and requires you to disassemble the whole iPad. If you are going to do this yourself, make sure to get the right tools and kits and follow the guide step by step. I would get the Essential Electronics Toolkit to get most of the tools needed. I hope you can fix that iPad! -Alex