Turned on Friends on Nick at Night tonight and noticed immediately they were at the end of Season 6, when Rachel's hair is at its longest and fullest. It's probably extensions, but it's spectacular. I searched Google to find out more and came across this is article by a young woman trying to replicate the 1994 Rachel look in 2020.

yahoo.com/lifestyle/rachel-greens-hair-season-6-215945867.html

Enjoy!