They’re the same thing.
The Relationships tabs lists all different relationships types, allowing you to specify to which and from which item types each relationship type is allowed, and whether an inverse relationship is implied or not.
The Relationships panel in the Item Types tab is a shortcut to edit the relationships for that specific item type, and changes to either will reflect in the other.
The “Relates To” relationship is the default always-available un-deletable relationship that exists within your system to let you put any two things in relation without needing a specific relationship type for it. Good for one-offs or very general relations that don’t really warrant a label. If you have no need for it, you don’t have to use it. But you can’t delete it.