Sparkling wine is all about creating secondary fermentation. With the right mix of yeast, sugar & wine in a closed environment like a bottle, still wines become bubbly as the carbon dioxide released by the yeast has nowhere to go. What makes a good or bad sparkling is where it happens & how long.
Champagne or traditional method is the most labor-intensive process, happening inside bottle, with a minimum lees-aging (where the sediment is left in the bottle). Lees are removed by sitting the bottle upside down and frozen, forcing the sediment out when uncapped, before a mixture of sugar and wine is added & corked. This process creates richness & complexity, used in Champagne, Cava, and Cremant
Charmat is a less expensive way where secondary fermentation happens in tanks. Fermentation stops when the winemaker cools the tank, afterwhich the wine is filtered & bottled. This method enhances clean fruit aromatics to create easy drinking wine, often used for Processo
Ancestral method is what's used today to create pet-nat. Instread of creating a secondary fermentation, the first fermentation is allowed to continue in bottle, resulting in a cloudy, earthy, textured wine