The Entity struct identifies an entity. If you need to access ComponentData on another Entity, the only stable way of referencing that ComponentData is via the Entity ID. EntityManager provides a simple get & set ComponentData API for it.
Entity myEntity = ...;
var position = EntityManager.GetComponentData<LocalPosition>(entity);
...
EntityManager.SetComponentData(entity, position);However, EntityManager can't be used in a C# job. ComponentDataFromEntity gives you a simple API that can also be safely used in a job.
// ComponentDataFromEntity can be automatically injected
[Inject]
ComponentDataFromEntity<LocalPosition> m_LocalPositions;
Entity myEntity = ...;
var position = m_LocalPositions[myEntity];