The FamilySearch API is defined in terms of resources, not endpoints. To consume the API, links are employed at runtime to discover all resources. Hard-coding endpoint URIs is unnecessary. You shouldn’t ever have to plug in identifiers to a URI template to access resources.
The only thing your client needs is the URI to a collection, such as FamilySearch Family Tree.
Think of the collection resource as the "index page" for the FamilySearch API. By retrieving the collection resource, your client will be able to follow links to all other resources available on the platform. For an explanation of how changes regarding resources are handled, see API Evolution guide.
An unauthenticated request to the collection resource will return the links for authentication. After authenticating, the collection resource will return links for viewing the current person, searching for a person, creating a person, creating relationships, and other links for getting started.