The rk3399 is an ARM architecture processor on a tv set top
box.
But rather than watching movies and playing games, I wanted to install Debian onto the box so it works like a computer.

The Debian page shows that a .iso image exists for arm64.
So how exactly can I install debian onto the ROM of the set top box?
The debian page seems to be saying that the arm64 .iso just needs to be burnt onto an sd card.
Is that correct?

I already have debian 9 on usb and the set top box has a usb interface.
How can I get the box to boot from usb?
Looking on the net, there's loads of stuff from developers compiling and optimising the board or the OS once it's installed.
Do I have to do the same thing?

I have found that installing a linux distro onto the rk3399 ARM chip is not yet ready for mainstream consumers.

There is a Kickstarter project where the rk3399 is used as a single board computer running on Ubuntu.
I think the board is used as a developer kit at the moment.

If the single board computer can run Ubuntu - then it should run debian with no problems.
I'm not sure if the board is optimised yet to run all apps on an OS without problems.

I'm not a developer so I'm not gonna install debian-arm64.iso and start tweaking it.
Hopefully we might see a debian arm64 SBC in the future.