For styling, we use the BEM syntax in addition to using an Atomic Design folder structure.
The styles/globals folder is designed to handle things like variables, animations, resets, default element styles, etc.
If a class is entended for a simple DOM element that won't have any child elements (links, buttons, paragraphs), that class belongs in the styles/atoms folder. At atom is the smallest building block possible.
If a class is entended to have child DOM elements, that class belongs in the styles/molecules folder. A molecule can contain and style any number of atoms or child elements.
The full Atomic Design pattern can also make use of additional folders. They can be useful for larger projects - such as organisms, pages, etc. However, atoms and molecules are usually the best starting point to avoid confusion.