USB that serves as an HID and a storage device

[MuEta]

Hello. I'm working on a project that would include a USB drive that contains a program that the computer user would download and install. The USB drive could also, on the press of a physical button, execute a command (through the HID interface) that would start a part of that program. I understand that Teensy may be able to solve this problem, but I have found no documentation that helps me to implement this, and I have very little hardware knowledge. I have worked with C, C++, and C# though, so I have the software knowledge.

Any help would be appreciated.

[MuEta]

Ok. I've just found out about composite USB devices, and specifically this post: https://www.microchip.com/forums/m725554.aspx , but I have NO IDEA what to do with this knowledge. Where do I even start? My usual practice of Googling until I find something isn't doing me any good.