Ecto.Adapters.DynamoDB.insert

You're seeing just the function insert, go back to Ecto.Adapters.DynamoDB module for more information.
Link to this function

insert(repo_meta, schema_meta, fields, on_conflict, returning, opts)

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Inserts a single new struct in the data store.

Autogenerate

The primary key will be automatically included in returning if the field has type :id or :binary_id and no value was set by the developer or none was autogenerated by the adapter.