This is not exactly a question, but more of an answer, because I can't seem to find details on the documentations regarding this scenario.

The example is based on the tutorial on Odoo's documentation

card.xml

How to add the slots would be:

export class Card extends Component {
    static template = "awesome_owl.card";
    static props = {
        title: String,
        slots: {
            type: Object,
            shape: {
                default: true,
                second: {
                    type: true,
                    optional: true,
                },
            }
        }
    }
}