There is no built-in endpoint for sites on a multisite network. As can be seen in the documentation, the built-in endpoints are:
+----------------+-------------------+
| Posts | /wp/v2/posts |
+----------------+-------------------+
| Post Revisions | /wp/v2/revisions |
+----------------+-------------------+
| Categories | /wp/v2/categories |
+----------------+-------------------+
| Tags | /wp/v2/tags |
+----------------+-------------------+
| Pages | /wp/v2/pages |
+----------------+-------------------+
| Comments | /wp/v2/comments |
+----------------+-------------------+
| Taxonomies | /wp/v2/taxonomies |
+----------------+-------------------+
| Media | /wp/v2/media |
+----------------+-------------------+
| Users | /wp/v2/users |
+----------------+-------------------+
| Post Types | /wp/v2/types |
+----------------+-------------------+
| Post Statuses | /wp/v2/statuses |
+----------------+-------------------+
| Settings | /wp/v2/settings |
+----------------+-------------------+
Accessing sites via a REST API endpoint will require a plugin. Either one you’d write yourself, or one that’s already available.