understanding useEffect concept in react code example

Example 1: react useeffect

import React, { useEffect } from 'react';
	
	const [data,setData]=useState()
    useEffect(() => {
        fetch(`put your url/api request in here`)
        .then(res=>res.json())
        .then(json => setData(json))
    },[])

Example 2: react useEffect

import React, { useEffect, useState } from 'react';
import ReactDOM from 'react-dom';

function LifecycleDemo() {
  // It takes a function
  useEffect(() => {
    // This gets called after every render, by default
    // (the first one, and every one after that)
    console.log('render!');

    // If you want to implement componentWillUnmount,
    // return a function from here, and React will call
    // it prior to unmounting.
    return () => console.log('unmounting...');
  }, [ // dependencies to watch = leave blank to run once or you will get a stack overflow  ]);

  return "I'm a lifecycle demo";
}

function App() {
  // Set up a piece of state, just so that we have
  // a way to trigger a re-render.
  const [random, setRandom] = useState(Math.random());

  // Set up another piece of state to keep track of
  // whether the LifecycleDemo is shown or hidden
  const [mounted, setMounted] = useState(true);

  // This function will change the random number,
  // and trigger a re-render (in the console,
  // you'll see a "render!" from LifecycleDemo)
  const reRender = () => setRandom(Math.random());

  // This function will unmount and re-mount the
  // LifecycleDemo, so you can see its cleanup function
  // being called.
  const toggle = () => setMounted(!mounted);

  return (
    <>
      <button onClick={reRender}>Re-render</button>
      <button onClick={toggle}>Show/Hide LifecycleDemo</button>
      {mounted && <LifecycleDemo/>}
    </>
  );
}

ReactDOM.render(<App/>, document.querySelector('#root'));

Example 3: react useEffect

import React, { useEffect } from 'react';

export const App: React.FC = () => {
  
  useEffect(() => {
        
  }, [/*Here can enter some value to call again the content inside useEffect*/])
  
  return (
    <div>Use Effect!</div>
  );
}

Example 4: how to use useeffect

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:  
  useEffect(() => {    
  	// Update the document title using the browser API    
  	document.title = `You clicked ${count} times`;  
  });
  
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

Example 5: react useeffect

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:  useEffect(() => {    // Update the document title using the browser API    document.title = `You clicked ${count} times`;  });
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}