They’re usually done automatically by a program that includes people who have already input their family trees. Once everyone is in, then it’s just a matter of running an algorithm over the tree to pull out the relationship.
FamilyTreeMaker, which I use personally, does that for everyone I’ve put into the program. Because I use my tree for research, I have 25,000 individuals in it. So I frequently want to ask how two people are related.
But something like the WikiTree has IMMENSE numbers of people to run the algorithm on. That last box is encouraging people to run it for themselves to see if they’re related SOMEHOW to a Rothchild.