How can I build a project (e.g. using idf.py build) but with a specific sdkconfig, like sdkconfig.test? Is it hardcoded to sdkconfig.defaults if present?

Hello everyone. I'm a bit puzzled with the boot sequence.
If I use the esptool.py, how is the sequence of flashing a firmware? Does the ROM bootloader need GPIO0 tied low to kick in? How is replaced this bootloader with the flasher stub?