>>5553154 (Q lb)
A free phone operating system will probably get you a long way to getting on every portable device on the planet. Then you just need Apple.
For other devices you need Microsoft, and the tricky one, Linux. But it's been many years since I've read the kernel code…
Then there's the hardware. The Intel CPU is basically a machine to run CPU code now, so would be easy to put nefarious stuff in there, not to mention any other piece silicon anywhere.
What I don't get is how you get data out again. Small amounts, easy, but I watch my router logs, I don't see anything that could account for large blobs of binary data from dozens of devices being transferred.