Although Firefox is still the default browser on Debian OS, you can also find the Chromium browser in the repositories. Chromium is the open source project on which Google has built the Chrome browser. It is also preferred by many Linux users as it provides almost the same features as Google Chrome.
So before, Chromium used Google as default search engine in Debian. However, Debian will use DuckDuckGo as Chromium's default search engine.
It all started when the bug report was submitted #956012 in April 2020. This bug states that DuckDuckGo is used as the default search engine for the Chromium package. As you can see the decision was not made in haste as it took maintainers more than two years to close the bug report.
The reason for this change is also stated in official announcement package update.
Change default search engine to DuckDuckGo for privacy reasons. Set a different search engine in Settings -> Search Engine (closes: #956012).