Privacy Sandbox

The Privacy Sandbox is an initiative led by Google to create web standards for websites to access user information without compromising privacy. Its core purpose is to facilitate online advertising without the use of third-party cookies. The initiative includes Federated Learning of Cohorts as well as other proposed technologies, many of which have bird-themed names. It was announced in August 2020.
The initiative has been described as anti-competitive and has generated an antitrust response.
Model.
Proposals in the Privacy Sandbox are based on advertising through cohorts rather than to individuals. They generally place the web browser in control of the user's privacy, moving some of the data collection and processing that facilitates advertising onto the user's device itself. There are three focuses within the Privacy Sandbox initiative: replacing the functionality of cross-site tracking, removing third-party cookies, and mitigating the risk of device fingerprinting.
Proposals.
Federated Learning of Cohorts (FLoC).
The Federated Learning of Cohorts algorithm analyzes users' online activity within the browser, and generates a "cohort ID" using the SimHash algorithm to group a given user with other users who access similar content.
Proposals for serving advertisements.
TURTLEDOVE.
TURTLEDOVE, which stands for "Two Uncorrelated Requests, Then Locally-Executed Decision On Victory", is a framework proposed by Google to serve ads through the browser.
SPARROW.
SPARROW, which stands for "Secure Private Advertising, Remotely Run On Webserver", is a proposal made by advertising company Criteo in response to Google's TURTLEDOVE. It would place a third party in control of parts of the ad service process, so that the browser company would not have as much control.
Dovekey.
Dovekey is a proposal made by Google in response to both SPARROW and TURTLEDOVE. It places the final part of the ad service process in the browser, but uses a third party server for key-value association.
PARRROT.
PARRROT is a proposal by advertising company Magnite in response to TURTLEDOVE. It places ad publishers in control of the ad service process through code embedded in their websites.
Antitrust concerns.
In January 2021, the Competition and Markets Authority in the United Kingdom announced plans to investigate the Privacy Sandbox initiative, with a focus on its potential impacts on both publishers and users.
In March 2021, 15 attorneys general of U.S. states and Puerto Rico amended an antitrust complaint filed in December; the updated complaint says that Google Chrome's phase-out of third-party cookies in 2022 will "disable the primary cookie-tracking technology almost all non-Google publishers currently use to track users and target ads. Then [...] Chrome, will offer [...] new and alternative tracking mechanisms [...] dubbed Privacy Sandbox. Overall, the changes are anticompetitive". The lawsuit suggests that the proposed changes in the Privacy Sandbox would effectively require advertisers to use Google as a middleman in order to advertise.