To use Modal you need to place Modal / Trigger and Modal / Wrapper symbols on your page.
Use components to build interfaces. Search the component's name in the Component panel (Shift + A) or use quick search (Ctrl + E). Once you added a Component to your page unlink it to make changes. Right-click on the symbol’s label and choose unlink from symbol. This is the rule. We use overrides only as an exception for simple components that you'd rather want to re-style on the specific page within Style Guide (buttons, toggles etc. - mostly atoms).
Modals are used to reveal critical information, show information without losing context, or when the system requires a user response. Modals can also fragment a complex workflow into simpler steps and should serve a single purpose dedicated to completing the user’s task.