In the original MVC architecture, the fundamental idea was that the model was responsible for storing the application state, a view was responsible for rendering output to the user, and a controller was responsible for responding to user interactions.
The model can be completely unaware of any specific views or controllers. It only needs to provide an interface allowing views to observe the current state and controllers to update that state.
In practice, views and controllers usually aren’t independent and instead come as a pair. This is because most modern UIs use some kind of event-driven architecture where user interactions are indicated by events from some component rendered by the view that the controller then handles.
My go-to example to understand why this architecture is helpful is a UI that features a table showing some names and a count for each, alongside a chart visualising that data graphically. Here you would have a model that stores the names and counts as pure data, and you would have two view+controller pairs, one managing the table and one the chart. Each view observes the model and renders an updated table or chart when the model state changes. Each controller responds to user interactions that perhaps edit a name or change its count — whether by typing a new value as text in an editable table cell or by dragging somewhere relevant in the chart — by telling the model to update its state to match (which in turn causes all views observing the model to refresh, without any further action from whichever controller happened to be handling that user interaction).
In practical terms for a React application, we might implement this with a simple object/Map somewhere that holds the names and values (our “model”) and two top-level React components that each get rendered once into some appropriate container within the page. Each component would have props to pass in (a) the current state and (b) any functions to be called when the user makes a change. Then you just write some simple glue logic in plain old JavaScript/TypeScript that handles keeping track of observers of the model, registering an observer for each top-level component that causes it rerender when the state changes, and providing a handler for each type of change the user is allowed to make that updates the state and then notifies the observers.
There are lots of variations on this theme, for example once you start needing more complicated business logic to interpret a user interaction and decide what state change is required or you need to synchronise your front-end model state with some remote service. However, you can scale a very long way with the basic principle that you hold your application state as pure data in a model that doesn’t know anything about any specific user interface or remote service and instead provides an interface for any other modules in the system to observe and/or update that state.