Does anyone know how signal boosters work?

If I understand, you're asking about either WiFi or Fiber long haul boosters.

Tip: You'll likely require many millions to effectively develop, warehouse, market this type of device.

You'll also be competing with companies with multi-million per year marketing budgets.

If I were going to do this... first, I'd think twice as this will be a very difficult business with a high likelihood of failure, unless you have an unlimited budget available.

If I did have an unlimited budget, I'd likely focus on tech supporting WiFi only (no Fiber) with 802.11ac running over 802.11s (mesh network) where I could acquire low cost hardware, likely manufactured in China.

Ensuring https://dd-wrt.com would run on the hardware.

Then I'd do some value add to DD-WRT functionality, where the value add would be the basis of some unique marketing message.

And... Wow... There are many far easier ways to generate income with far more likely success.