Anonymisation means removing identifiability from footage before it is shared or published. In practice that means blurring faces and vehicle number plates, cropping the frame so people fall outside it, or simply dropping the shots in which someone is recognisable.
Done properly, the material no longer contains personal data and therefore falls outside the scope of GDPR's rules. That is exactly why it is the standard fix for incidental bystanders in drone footage — the people can no longer be identified, and the video can be published safely.