Planescape and Ravnica are two very different settings. The central core areas of Planescape is Sigil, but it also includes the inner, outer, Astral, and energy planes, Sigil is mostly home base and an area where adventures cam occur. Planescape is no more only Sigil, then FR is only Waterdeep.
Ravnica on the other hand is both the name of the capital city/metropolis, the name of the world/Ecumenopolis, and the plane.
I don't think Setting goes beyond the Ecumenopolis of Ravnica, so no trips to the plane of fire normally (well you can in the D&D version, but then you've left the Setting for the metasetting).
In fact that is a good way to explain the difference between Planescape and Ravnica, Planescape is a metasetting, the connective tissue between other Settings, Ravnica is a Setting, and the Forgotten Realms is a hungry setting that devoured enough other setting and elements that it's evolved into a Metasetting (regions in Faerun, like Old Empires, Cold Lands, Lands of Intrigue, Moonshaes, Moonsea, Swordcoast and the North, Unapproachable East, Utter East, Western Heartlands, Shining South, Chult, The Vast, The Upper Underdark, Lower Underdark, Middle Underdark, FR Feywild, FR Shadowfell, FR Domains of Gods, The Great Rift, Evermeet are basically their own Setting functionally, that interact, then you get to actual official subsettings Kara Tur, Maztica, Zakhara, and possibly in the future Abeir).