Template, slot, and shadow - web.dev

mdn slot When the slot's containing component gets rendered, the slot is rendered with the custom element's child that has a matching slot attribute. A named slot is a slot> element. An element with a slot attribute is assned to the slot created by the slot> element whose name attribute's value matches that slot attribute's value.

mdnsport, The slot> tag is a placeholder inside a web component that you can fill with your own markup, which lets you create separate DOM trees and present them together. Use the template>.