My question is about the Ancestry Genealogy tests (Autosomal DNA tests) being offered.

Suppose a consume DNA testing service company, like 23andMe/Ancestry.com/FamilyTreeDNA, extracts two people's genetic information.

How will the company test the relative relationship of two individuals? Will they use some algorithms to detect the identity-by-descent (IBD) segments?

Could you please introduce me some effective but also efficient/simple algorithms for ancestry/genealogy test? So far, I only know detection by IBD.