spring-boot-starter-data-elasticsearch
internally can use the transport(soon to be deprecated in ES 8.X)
or rest-high-level-client
Please refer elasticsearch client section for more information and how to configure them.
And from the same link :
Spring data Elasticsearch operates upon an Elasticsearch client that
is connected to a single Elasticsearch node or a cluster. Although the
Elasticsearch Client can be used to work with the cluster,
applications using Spring Data Elasticsearch normally use the higher
level abstractions of Elasticsearch Operations and Elasticsearch
Repositories.
Bottom line is that you can directly use rest-high-level
client in your spring boot application but if you want more abstraction then you can use the spring-boot-starter-data-elasticsearch
dependency and use its method which provides more abstraction although internally it would use the client offered by Elasticsearch.