I have had broody hens adopt chicks. But, yes, the hen has to be broody first. Mine hadn't been sitting for 21 days, but they had been sitting long enough for me to know that they were serious about it. I don't remember exactly how long they had been sitting but I would say 10 days at least.
Yes, I did slip the chicks in under the hen at night with out any lights on. I didn't bother taking out the eggs/ fake eggs at that time. I just wanted to get the chicks under her. I would only do a couple at a time and then go out later ( 30 to 60 minutes) and put a couple more under her. I have even added some more chicks after 3 days, but she was not a first time Momma.
I agree with going out and checking to make sure they were not rejecting the chick(s). I did have one broody that accepted all the chicks but 1. For some reason it did not like a fuzzy black chick, but fortunately I had a 2nd broody and she accepted that chick.
The pros to having broody reared chicks is that they integrate with the flock MUCH EASIER! Momma doesn't let the rest of the flock pick on her chicks! Even our lowest ranked, most docile hen became a fierce Momma! The chicks learned all they needed to know from Momma hen. The broody raised chicks learn much faster how to get up on the ramp to get into the coop. The brooder raised chicks ( my brooder is out in the coop) don't figure out the ramp until they are much older even though they see the other chickens going up and down it.

The Momma broody keeps the chicks warm, so there is no need for heat lamps!! Broody hen raised chicks also go outside sooner and learn about eating grit and bugs and where and how to find food and water sooner because Momma teaches it all to them. She also teaches them about danger and what to do when danger is near.