You could think about it this way… Bootstrap and Tailwind are frameworks that have a collection of “helper classes”. Use the Element Properties panel to apply these helper classes. That’s where you’ll find things like gradients, spacing, text colours, and so on., or you want to make your own rules (for things helper classes don’t cover), then use the Styles panel to create and manage custom classes.
If you want further explanations about the frameworks themselves and their respective helper classes, both have well documented websites.