In summary, types are just a way to make our life easier in the process of development with TypeScript and JavaScript.
Because JavaScript is a dynamic language and doesn't have a static type, all the types and interfaces declared in TypeScript are strictly used just by TypeScript. This helps the compiler catch errors, warnings, and the language server to help the integrated development environment (IDE) on the development process to analyze your code as it is being written.
This is a basic introduction to TypeScript, covering the basics of the typed language, and how to understand and use it. There is much more to learn about its use, such as generics, modules, namespaces, and so on.
With this introduction, you can understand how the new Vue 3 core works and how to use the basics of TypeScript in your project, and take advantage of the typed language on your project.
There is always more knowledge to find on TypeScript, as it is a growing "language" on top of JavaScript and has a growing community.
Don't forget to look at the TypeScript documentation to find out more about it and how it can improve your code from now on.