Error boundaries in React should be placed at the top level of the component tree, as close to the root of the application as possible. This ensures that errors are caught and handled as early as possible in the rendering process, and prevents them from affecting other components or causing cascading errors.

In React, state updates are merged with the existing state using the spread operator. When setState() is called, React updates the state object by merging the new state object with the existing state object. This allows components to update specific properties of the state object without overwriting the entire object.

Portals in React are a way to render a child component outside of its parent component. This is useful in cases where you need to render a child component in a different part of the DOM hierarchy, such as when creating modals or tooltips.

When a component is re-rendered in React, the order of methods that are called is as follows: render(), componentDidUpdate(), and then componentDidMount(). The render() method is called to update the component's UI based on any changes in props or state, followed by componentDidUpdate() to handle any side effects that may have occurred as a result of the update. Finally, componentDidMount() is called to perform any additional setup that may be necessary for the updated component.

In React, components can be created using the React.createElement method or JSX syntax. React.createElement is a lower-level API for creating components, while JSX is a syntax extension that allows developers to write HTML-like code in JavaScript. 

In React, props are immutable and cannot be changed directly. This is because React is designed to be a one-way data flow, where data is passed down from parent components to child components through props. If you need to update the data, you should do so in the parent component and pass the updated data down as props to the child components.

React memo is a higher-order function that creates a memoized version of a component. This memoized version can help improve performance by avoiding unnecessary re-renders of the component. The memo function works by caching the result of the component's render method and comparing it to the previous result. If the result is the same, the component is not re-rendered.

Flow is a type checking tool for JavaScript that is commonly used in React and React Native applications. Flow allows you to add static types to your JavaScript code, which can help to catch errors at compile-time instead of at runtime. Flow is particularly useful in large codebases where it can be difficult to keep track of all the variables and function calls.

Components and containers in React Redux have different responsibilities. Components are responsible for rendering UI elements and receive props from their parent components, while containers are connected to the Redux store and manage the application state.

The main difference between state and props in React is that state is managed within a component, while props are passed down from a parent component. State is used to manage a component's internal data, which can change over time, while props are used to customize a component's behavior and appearance.