How can I load the transitions for a particular workflow?

The easiest way to do this is through the workflow's type plugin, specifically the getTransitions() method.

// First load a workflow, for example:
$workflow = \Drupal::service('content_moderation.moderation_information')
  ->getWorkflowForEntity($entity);

// Then get the workflow's type plugin.
/** @var \Drupal\workflows\WorkflowTypeInterface $workflow_type_plugin */
$workflow_type_plugin = $workflow->getTypePlugin();

// Finally, load the transitions.
$workflow_transitions = $workflow_type_plugin->getTransitions();

Alternatively, it may be more useful to only get the valid transitions for an entity, i.e. only transitions allowed for the entity from its current state, for the current user. The getValidTransitions() method from the content_moderation.state_transition_validation service does this.

/** @var \Drupal\content_moderation\StateTransitionValidationInterface $transition_validation */
$transition_validation = \Drupal::service('content_moderation.state_transition_validation');
$valid_transitions = $transition_validation
  ->getValidTransitions($entity, \Drupal::currentUser());