I want to program multiple arduino at once
Arduino, the company, may sell on enough volume to make it worthwhile to get the microcontroller manufacturer to program the microcontrollers durin the production process
This wouldn't cost them extra if they already get the bootloader pre-programmed. I've read programming ICs is a service available to high volume customers.
I guess low volume makers use fixtures that use the ICSP headers, pogo sticks or IC clips and which trigger the programming on contact using a microswitch etc.
I suspect Adafruit do something of this sort, see GIFs and video discussing testing at
https://www.adafruit.com/product/2458