There are two ways to build & flash ExpressLRS. The recommended way is to use ExpressLRS Configurator. For advanced users who want to modify the code, you can setup the development toolchain.
The targets for each of the supported hardware can be seen on the individual device pages. Use the navigation menu on the left to get to the specific hardware page.
On the next page, you will find the different Firmware Options you can set along with their explanations.