I found only these two statements from BOL regarding this Initial Data Synchronization method:
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.
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
On secondary replica
My question is related to the working nature of automatic seeding, namely where does taken backup is stored before restoring on the secondary replica?