If im right (been a long time since i rooted a modern android), the pixel uses an A/B immutable partition scheme, so each update is a full new base OS uploaded in, then user data is moved over.
Whether it uses physical A/B, virtual A/B, the updates are delivered as deltas from the previous version. Typically monthly updates range between several megabytes and tens of megabytes. Hundreds of megabytes implies a lot of system changes. The delta is used to create a full new base OS partition by combining the currently active partition with the delta. The result is written to the inactive partition. On Pixel, there are no physical A/B partitions anymore. Instead Linux logical volumes and snapshots are used, but the effect is the same as having physical A/B during update. Post update the inactive "partition" is removed and there's a single active volume till the next update.