An ecumenical council has more authority than a synod. Synods have long been around, but in the RCC have been as delegated to handle a local issue specific to one area, such as the US meeting at Baltimore. Since Vatican II, synods of bishops from around the world have gathered at the Vatican, to handle a special assigned topic.
I believe the reason these modern synods take place is that it was felt there was too long a time between Trent, and incomplete Vatican I, and Vatican II. The world was changing, so a special office was established at the Vatican for future synods. But they are advisory only.
The “Council” or “synod” at Jerusalem is in a somewhat separate category since it is in the Bible. Many Protestants who reject any “Council” authority do accept this council, not because it is a council but because it is in the Bible. Other Protestants accept the authority of certain, always the earliest, councils. Eastern Orthodox accept the authority of certain councils and synods, not necessarily the same as Rome.
The canon of Scripture was actually first approved by a series of local Councils in Carthage, Hippo, and Rome.
The decision to “authorize” these councils, and approve their recommendations for the NT canon, was made by the Magisterium. I suppose the Gnostic Christians had councils that “approved” other books for the New Testament, but you won’t find those books in your Catholic bible.