React rendering basics

Rendering introduction

Rendering is the process during which React moves down the component tree starting at the root, looking for all the components flagged for update, asking them to describe their desired UI structure based on the current combination of props and state. For each flagged component, React will call its render() method (for class components) or FunctionComponent() (for function components), and save the output produced after converting the JSX result into a plain JS object, using React.createElement().

After collecting the render output from the entire component tree, React will diff the new tree (the virtual DOM) with the current DOM tree and collect the list of changes that need to be made to the DOM to produce the desired UI structure. After this process, known as reconciliation, React applies all the calculated changes to the DOM.
Render and commit phases

Conceptually, this work is divided into two phases:

Render phase: rendering components, calculating changes
Commit phase: applying the changes to the DOM

After the commit phase is complete, React will run componentDidMount and componentDidUpdate lifecycle methods, as well as useLayoutEffect and, after a short timeout, useEffect hooks.

Two key takeaways here are the following:

Rendering is not the same as updating the DOM
A component may be rendered without any visible changes

Rendering reasons

After the initial render has completed, there are a few different things that will cause a re-render:

this.setState() (class components)
this.forceUpdate() (class components)
useState() setters (function components)
useReducer() dispatches (function components)
ReactDOM.render() again (on the root component)

Rendering behavior

React's default behavior is to recursively render all child components inside of it when a parent component is rendered. This means that it does not care if a component's props have changed - as long as the parent component rendered, its children will render unconditionally.

To put this another way, calling setState() in the root component without any other changes, will cause React to re-render every single component in the component tree. Most likely, most of the components will return the exact same render output as the last render, meaning React will not have to make any changes to the DOM, but the rendering and diffing calculations will be performed regardless, taking time and effort.