There are four types of builders, plus there is an optional release deploy step. I don’t know what you are trying to do, but I suspect that what you want is the release deploy step.
The four types of builders are depot, fly legacy, rchab, and local. fly legacy and rchab are basically the same thing, but by launching it yourself you can have greater control.
Release commands are defined in fly.toml, and are run on an ephemeral machine with your image and secrets, and on your network. Even if you are deploying to 50 servers, release commands are run exactly once.