how to create app bar with flex box code example

Example: responsive flexbox navbar

/*
*               *
* Menu / Header *
* Menu / Header *
* Menu / Header *
* Menu / Header *
*               *
*/

nav {
  background: #222;
  padding: 5px 20px;
}
ul {
  list-style-type: none;
}
a {
  color: white;
  text-decoration: none;
}
a:hover {
  text-decoration: underline;
}
.logo a:hover {
  text-decoration: none;
}
.menu li {
  font-size: 16px;
  padding: 15px 5px;
  white-space: nowrap;
}
.logo a,
.toggle a {
  font-size: 20px;
}
.button.secondary {
  border-bottom: 1px #444 solid;
}
/* Mobile menu */
.menu {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  align-items: center;
}
.toggle {
  order: 1;
}
.item.button {
  order: 2;
}
.item {
  width: 100%;
  text-align: center;
  order: 3;
  display: none;
}
.item.active {
  display: block;
  background-color: rgb(192, 79, 79);
  border-radius: 4px;
}
/* Tablet menu */
@media all and (min-width: 600px) {
  .menu {
    justify-content: center;
  }
  .logo {
    flex: 1;
  }
  .toggle {
    flex: 1;
    text-align: right;
  }
  .item.button {
    width: auto;
    order: 1;
    display: block;
  }
  .toggle {
    order: 2;
  }
  .button.secondary {
    border: 0;
  }
  .button a {
    padding: 7.5px 15px;
    background: teal;
    border: 1px #006d6d solid;
  }
  .button.secondary a {
    background: transparent;
  }
  .button a:hover {
    text-decoration: none;
  }
  .button:not(.secondary) a:hover {
    background: #006d6d;
    border-color: #005959;
  }
  .button.secondary a:hover {
    color: #ddd;
  }
}


/* Desktop menu */
@media all and (min-width: 900px) {
  .item {
    display: block;
    width: auto;
  }
  .toggle {
    display: none;
  }
  .logo {
    order: 0;
  }
  .item {
    order: 1;
  }
  .button {
    order: 2;
  }
  .menu li {
    padding: 15px 10px;
  }
  .menu li.button {
    padding-right: 0;
  }
}

/*
*               *
! Menu / Header SLUT *
! Menu / Header SLUT *
! Menu / Header SLUT *
! Menu / Header SLUT *
*               *
*/