Here:

When a user interacts with an ad or advertiser, an event is logged in the browser in the form of a value. That value is then split into partial, indecipherable pieces and then encrypted. Each piece is addressed to a different entity — one to Divvi Up at ISRG and one to Mozilla — so that no single entity is ever in possession of both pieces. (...) As an additional protection, the pieces are submitted to Divvi Up and Mozilla using an Oblivious HTTP relay operated by a third organisation (Fastly). This ensures that Divvi Up and Mozilla do not even learn the IP address of the indecipherable piece they receive.