In Create React App, you can include polyfills to support older browsers by using the "core-js" library, the "@babel/preset-env" package, or the "react-app-polyfill" package. These packages provide polyfills for various ES6+ features and browser APIs, and can be configured in the "babel.config.js" or "webpack.config.js" files.

React has several advantages, including improved performance, reusable components, and good community support. React's use of a virtual DOM and its focus on component-based architecture help improve performance and make it easier to develop complex UI components. React's component-based architecture also makes it easier to create reusable components that can be used across multiple projects. React has a large and active community of developers, which provides good support and resources for learning and development.

Keyed Fragments are a feature in React that allows elements to be grouped together without adding extra nodes to the DOM. This can improve performance and reduce the complexity of the component hierarchy. Keyed Fragments are created by using the element and adding a unique key prop to each child element.

Relay is a library for managing network requests in React applications. It is often used in conjunction with GraphQL APIs and provides a number of features for managing data fetching and caching, such as automatic batching, optimistic updates, and pagination. Relay also provides a way to declare data dependencies for your components, which can help to reduce unnecessary re-renders.

To conditionally render components in React, you can use the ternary operator in JSX. This allows you to specify a condition, and render one component if the condition is true, and another component if the condition is false. You can also use other conditional statements, such as "if" statements or switch statements, outside of the JSX to determine which component to render.

In React context, a consumer is a component that consumes a context value. The consumer component allows child components to access the context value without the need to pass props explicitly. Consumers can be implemented using the useContext() hook or the Consumer component, which provides a render prop that can be used to consume the context value.

Formik is a utility library for working with forms in React. It provides a simple and flexible way to handle form validation, input masking, and submission handling. Formik also integrates with other popular form libraries, such as Yup and React-Select.

React Mixins are reusable code snippets that can be applied to multiple components in order to provide shared functionality. They are a way to encapsulate logic and behavior that can be used across multiple components, allowing for code reuse and reducing duplication. However, they are not recommended in modern versions of React and have been largely replaced by higher-order components and render props.

Loadable components are a way to load other components asynchronously in React. Loadable components allow components to be split into smaller chunks and loaded on-demand, improving performance and reducing initial load times. Loadable components are typically used with dynamic imports to enable asynchronous loading of code.

Higher-Order components (HOCs) are components that enhance the behavior of other components by adding additional functionality or props. HOCs take a component as input and return a new component that includes the additional behavior. This allows developers to reuse code and separate concerns in their applications.