+20% Production towards Medieval, Renaissance, and Industrial Era Wonders. Wonders grant 2x normal Tourism in every era.
Unique Units
Garde Imperiale (Industrial Melee unit), Château (Tile improvement, grants Culture, Gold, Appeal, and extra Culture if next to a Wonder)
There are two versions of Catherine de Medici in Civilization 6, but the Magnificence version is significantly stronger when it comes to getting fast Culture victories. Civ VI pairs this version of Catherine with the French civilization, which doubles the Tourism of Wonders and grants Production bonuses for building Wonders from the Medieval to Industrial eras.
This pairs extremely well with Catherine's Leader ability, which grants extra Culture for Luxury Resources next to either a Theater district or a Château, a unique French building. When placed next to a wonder, Châteaus generate extra yields of all types, including Appeal to nearby tiles.
She can also initiate a Court Festival in cities with a Theater Square, significantly boosting Culture and Tourism based on your copies of luxury resources in that city.
So, the strategy goes something like this – focus on building your Culture foundations in the early game, swap to Industry and Production to grab as many Wonders as you can, and harvest every Luxury Resource and trade for copies of Luxury Resources you already have to maximize the bonuses of the Court Festival.
It sounds like a lot to juggle, and compared to some simpler civs, Catherine de Medici (Magnificence) is definitely on the more complex side. But if you get lucky with your founding city placement next to lots of Luxury Resources, and can effectively snag useful Culture Wonders during the Medieval through Industrial eras, you will be significantly ahead by the time the Modern era rolls around.
Continue to build out your Theater Squares, snag Great Works, and run Court Festivals in your cities when they get copies of Luxury Resources, and you will be on route for one of the fastest Culture victories Civ VI players can get.