Install APK to firestick, fire tablet, fire tv from phone or PC, no ADB/USB required

How to sideload install APK on Amazon&Firestick, Fire TV Cube, and Fire Tablet devices? The Firestick lacks a USB port, while the Fire TV Cube has a micro USB port. However, it is not straightforward to install APK files from a USB device. Some may suggest using ADB debug commands, but that can be complicated for most users. Is there an easier way to do it without USB or ADB commands? Yes, there is. Watch this video to learn how to use the S2X File Manager to accomplish the task.