How is the default launcher set after a factory reset
If a ROM has multiple launchers, and one is not set by default in the default factory configuration, then after the Setup Wizard completes and the device attempts to go to the desktop, a pop-up appears asking you to select the Home application and it will give you an option to "Always do this".

You can select a launcher here, but until you select "Always do this" (or similar text) this pop-up selection window will appear every time the Home button is pressed.

This is common in custom ROMs which have multiple launchers pre-installed.