How CrudRepository deals with DB Connection from ConnectionPool

Question

I can't find any default implementation class for spring-data-commons crudrepository interface. We always implement CustomRepository extends CrudRepository<T, ID> and then @Autowire CustomRepository customRepository in the service class to call customRepository.save(s)..which always works.

I'd like to understand the low level details associated with any crudrepository method e.g. save(S);

1. When is the connection to underlying database opened or closed?
2. How the transaction boundaries are defined?
3. How does it work with connection-pool?
   
   Any pointers to the right resources would be really helpful.Please feel free to point me out if I'm missing something silly...