They're not really secret, they're standard practice. Not all that widely known, but that's because they're geeky trivia to most people. There are multiple microwave relay networks between Chicago and NYC. It's neat stuff.

I've worked for an options market maker, chasing nanoseconds of latency improvements. If you want racks in the exchange data center, you have to pay more to be on one side of the room vs. the other, because the latency improvement for being a couple hundred feet closer actually matters.

If you are really curious, I highly recommend this blog!

https://sniperinmahwah.wordpress.com/ (Many people who work in HFT are quite familiar with it.)

It's an entertaining and fascinating "view of the inside."

Some hft firms are rumored to be using short wave radio. Everything old is new again.

https://hackaday.com/2018/05/12/hft-on-hf-you-cant-beat-it-f...