How does Automatic Seeding work – Always On Availability Group


I found only these two statements from BOL regarding this Initial Data Synchronization method:

Link 1

Automatic seeding uses the log stream transport to stream the backup using VDI to the secondary replica for each database of the availability group using the configured endpoints.

Link 2

During automatic seeding, SQL Server performs a backup over the network for initialization.

Looking into SQL Server Error logs of both primary and secondary replicas, these two logs caught my attention:

On primary replica

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On secondary replica

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My question is related to the working nature of automatic seeding, namely where does taken backup is stored before restoring on the secondary replica?