Posicionando elementos com Flexbox em CSS

*{
      margin: 0;
      padding: 0;
      font-family: 'Open Sans', sans-serif;
  }
  

  ul{
      list-style: none;
      margin: 0;
      padding: 0;
  }
  ul li a{
      text-decoration: none;
  }
  div img{
      display: block;
      width: 100%;
  }
  .flex-container{
      display: flex;
      max-width: 992px;
      margin: auto;
      width: 100%;
      min-width: 320px;
  

  }
  header{
      background-color: #122A57;
      height: 100px;
      display: flex;
      align-items: center;
      color: #fff;
  }
  header .list-items{
      display: flex;
      max-width: 260px;
      width: 100%;
      justify-content: space-between;
      align-items: center;
  }
  .list-items li a{
      color: #fff;
  }
  header .menu{
      justify-content: space-between;
  }
  .apresentação{
      height: 100vh;
      align-items: center;
      justify-content: space-between;
  }
  .apresentação .texto-apresentação{
      min-height: 200px;
  

  }
  .texto-apresentação h1{
      color: #122A57;
      font-size: 48px;
      margin-bottom: 10px;
  

  }
  .btn{
      background-color: #122A57;
      color: #fff;
      text-align: center;
      border-radius: 30px;
      width: 220px;
      display: block;
      text-decoration: none;
      height: 50px;
      line-height: 50px;
      margin-top: 10px;
      margin-bottom: 10px;
  

  }
  #quem-somos{
      flex-direction: row-reverse;
      align-items: center;
      justify-content: space-between;
      
  

  }
  #quem-somos h2{
      font-size: 32px;
      color: #122A57;
      display: flex;
      margin-bottom: 20px;
  }
  

  #quem-somos h2::before{
  

      content: "";
      height: 50px;
      width: 5px;
      margin-right: 5px;
      background-color: #122A57;
      position: relative;
  }
  #quem-somos p{
      margin-bottom: 10px;
      width: 90%;
  }
  #quem-somos img{
      min-width: 420px;
  }
  .container-externo{
      background-color: #122A57;
      width: 100%;
  }
  #servicos{
      flex-direction: column;
      padding-top: 50px;
      padding-bottom: 100px;
  }
  #servicos h2{
      color: #fff;
      font-size: 32px;
      margin-bottom: 20px;
  }
  .list-servicos{
      display: flex;
      justify-content: space-between;
  }
  .list-servicos .item-servico{
      text-align: center;
  }
  .item-servico a{
      width: 220px;
      background-color: #fff;
      border-radius: 30px;
      height: 50px;
      text-align: center;
      margin-top: 20px;
      line-height: 50px;
      padding: 5px  10px;
      color: #122A57;
      font-size: 12px;
      text-decoration: none;
      font-weight: 700;
  }
  .item-servico p{
      font-weight: 700;
      font-size: 18px;
      color: #fff;
      margin-top: 20px;
  }
  .item-servico img{
      width: 80%;
      margin: auto;
  }
  #planos{
      flex-direction: column;
      min-height: 100vh;
      margin-top: 50px;
  }
  #planos h2{
      font-size: 32px;
      padding-top: 50px;
  }
  .list-planos{
      display: flex;
      align-items: flex-end;
      justify-content: space-between;
  }
  .item-plano{
      flex: 1;
      border: 5px solid #122A57;
      margin-right: 20px;
      padding: 10px;
      max-width: 240px;
  }
  .item-plano .btn{
      margin: auto;
      margin-bottom: 20px;
  }
  .item-plano h3{
      font-size: 24px;
      display: flex;
      flex-direction: column;
      text-align: center;
      margin-top: 20px;
  }
  .item-plano h3::after{
      content: "";
      background-color: #122A57;
      width: 100%;
      height: 3px;
      margin-top: 20px;
      margin-bottom: 10px;
  

  }
  .item-plano ul{
      padding: 10px 20px;
      display: flex;
      flex-direction: column;
      margin-bottom: 10px;
  }
  .item-plano ul li{
      display: flex;
      flex-wrap: nowrap;
      align-items: center;
  }
  .item-plano ul li::before{
      content: "";
      width: 10px;
      height: 10px;
      background-color: #122A57;
      margin-right: 5px;
  }
  footer{
      background-color: #122A57;
      height: 70px;
      display: flex;
      align-items: center;
  }
  footer .footer{
      justify-content: space-between;
      color: #fff;
  }
  /*mobile*/
  @media(max-width:992px){
      .flex-container{
          flex-direction: column;
      }
      .apresentação{
          flex-direction: column-reverse;
      }
      .apresentação .texto-apresentação{
          width: 100%;
      }
      #quem-somos{
          flex-direction: column-reverse;
      }
      #quem-somos img{
          min-width: 320px;
          margin: auto;
      }
      .list-servicos{
          flex-direction: column;
      }
      .item-servico img{
          width: 50%;
          margin: auto;
      }
      .list-planos{
          flex-direction: column;
          align-items: flex-start;
          margin-bottom: 20px
      }
      ;.list-planos .item-plano{
          max-width: 90%;
          margin: auto;
          height: 100%;
          margin-bottom: 20px;
      }
  

  }

[Flex Projeto](https://github.com/Sandradiotech/Flex_Projeto/tree/main/Flexbox)