With conversion modeling, Google can fill in gaps in measurement when users deny cookies.
Conversion modeling is using for this method observed conversions to predict unobserved conversions. This all happens without identifying an individual.
Google has described this process in 4 steps:
1. Ad interactions are separated into two groups
2. The observed group is divided into subgroups
3. The unobserved group is sorted into those same subgroups
4. Unobserved ad interactions and conversions are linked
For more information about how this works check: https://support.google.com/google-ads/answer/12443859