@Sarva, Pavan Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
As per this documentation, we don't have any child resources under SearchServices to create the indexer using either ARM/bicep/terraform template.
You can create an indexer through either Azure portal or Rest API or using .NET sdk only.
Also, you can refer to this sample GitHub to create a cognitive search service with cosmos db indexer in which indexer is getting created by calling the indexer REST API through PowerShell.
Refer to this documentation about the tips for better performance in cognitive search.
Feel free to reach back to me if you have any further questions on this.