With a Create React App (short CRA) you could place your video files in the public folder e.g. in a subfolder videos. Then you can access that static file as you did it in your code.

CRA will just copy the files to the final static page as mentioned in the docs.

If you don't use a CRA setup you need to copy your video to the final bundle e.g. by using Webpack.

import React, { Component } from 'react'
import ReactPlayer from 'react-player'

class Video extends Component {
    render () {
        return (
        <div className='player-wrapper'>
            <ReactPlayer
            className='react-player fixed-bottom'
            url= 'videos/demo_video.MP4'
            width='100%'
            height='100%'
            controls = {true}

            />
        </div>
        )
    }
}

export default Video;