grid vs box vs div material ui code example

Example: react mui grid

//basic grid layout
import React from 'react';
import { makeStyles } from '@material-ui/core/styles';
import Paper from '@material-ui/core/Paper';
import Grid from '@material-ui/core/Grid';

const useStyles = makeStyles((theme) => ({
  root: {
    flexGrow: 1,
  },
  paper: {
    padding: theme.spacing(1), //grid padding
    textAlign: 'center',
    color: theme.palette.text.secondary,
  },
}));

export default function NestedGrid() { //export default allows for other modules to 
  //import in the grid function. 
  //create class based upon class outside of export default. 
  const classes = useStyles();

  function FormRow() {
    return ( //return renders the grid
      
        
          item
        
        
          item
        
        
          item
        
      
    );
  } //end of function declaration/creation FormRow()
//usage of formrow element. The declaration above doesn't run.