const ELS_pages = document.querySelectorAll('.page');
const ELS_buttons = document.querySelectorAll('[data-page]');
const submit = document.querySelector('.submit');

const goToPage = (id) => {
  ELS_pages.forEach((EL, i) => {
    if (EL.id === id) {
      setTimeout(() => {
        EL.classList.remove('u-none');
      
        // THIS TIMEOUT IS IMPORTANT TO ALLOW BROWSER TO REPAINT AFTER CHANGING display PROPERTY. IF NOT, TRANSITION COULD NOT WORK.
        setTimeout(() => {
          EL.style.opacity = 1;
        }, 100);
      }, 1000);
    } else {
      EL.style.opacity = 0;
      
      setTimeout(() => {
        EL.classList.add('u-none');
      }, 1000);
    }
  });
};

ELS_buttons.forEach((EL) =>
  EL.addEventListener('click', () => {
    goToPage(EL.dataset.page);
  })
);

goToPage('page-main');

nav {
    display: flex;
}

nav a {
    color: #00f;
    padding: 5px 10px;
    cursor: pointer;
}

.page {
    opacity: 0;
    transition: opacity 1s ease-in-out;
}

.u-none {
    display: none;
}

<div class="main-container">
    <div class="page padding main u-none" id="page-main">
        <div class="modal hidden">
            <button class="btn-close-modal">&times;</button>
            <h2 class="modal-header">Login to your account</h2>
            <form class="modal-form">
                <div>
                    <label for="input-username">Username:</label>
                    <input id="input-username" type="text"/>
                </div>
                <div>
                    <label for="input-password">Password:</label>
                    <input id="input-password" type="text"/>
                </div>
                <button class="btn header-button">Login &rarr;</button>
            </form>
        </div>
        
        <div class="overlay hidden"></div>
        
        <nav class="header-nav">
            <a href="#" class="header-img">
                <svg viewBox="0 0 256 256" class="header-svg">
                    <path
                            class="path"
                            d="m 15.625507,46.199537 55.767886,-32.030352 55.623047,32.281229 -0.14485,64.311586 0.14485,-64.311586 55.76788,-32.030352 55.62305,32.281229 -0.14485,64.311579 -55.76788,32.03036 -55.62305,-32.28123 55.62305,32.28123 -0.14485,64.31158 -55.76788,32.03035 L 70.958866,207.10393 71.103708,142.79235 126.87159,110.762 71.103708,142.79235 15.480664,110.51112 Z"
                            style="
                   fill: none;
                   stroke: #000;
                   stroke-width: 13;
                   stroke-linejoin: round;
                   stroke-linecap: round;
                 "
                    />
                </svg>
            </a>
            <button
                    class="header-button login"
                    type="button"
                    data-page="page-login"
            >
                LOGIN
            </button>
        </nav>
        
        <div class="header-hero">
            <div class="circle1"></div>
            <div class="circle2"></div>
            <div class="header-text">
                <h1 class="header-text-description">
                    Make Banking Easy with Express
                </h1>
                <p class="header-text-texts">
                    Together we can make banking awesome!
                </p>
                <p class="header-text-texts">
                    Use our mobile app to your need today
                </p>
                <button class="header-button header-buttons" type="button">
                    <a href="">Services</a>
                </button>
            </div>
            <div class="header-image hide-for-desktop">
                <img src="./assets/hand.jpg" alt="" class="header-hand"/>
                <button class="header-image-1">Transfer</button>
                <button class="header-image-2">Loan</button>
            </div>
        </div>
    </div>

    <div class="page u-none" id="page-login">
        <nav>
            <a data-page="page-dashboard">User Settings</a>
            <a data-page="page-main">Logout</a>
        </nav>
        <h1>MAIN PAGE 1</h1>
        <form action="" name="login" method="POST" id="form-1">
            <label for="name-1">Name</label>
            <input type="text" id="name-1" required/>
            <label for="pin-1">Pin</label>
            <input type="text" id="pin-1" required/>
            <button type="submit" class="submit">Submit</button>
        </form>
    </div>

    <div class="page u-none" id="page-signup">
        <nav>
            <a data-page="page-settings">User Settings</a>
            <a data-page="page-login">Logout</a>
        </nav>
        <h1>MAIN PAGE 2</h1>
        <form action="" name="login" method="POST" id="form-id">
            <label for="name-2">Name</label>
            <input type="text" id="name-2" required/>
            <label for="pin-2">Pin</label>
            <input type="text" id="pin-2" required/>
            <button type="submit" class="submit">Submit</button>
        </form>
    </div>

    <div class="page u-none" id="page-dashboard">
        <nav>
            <a data-page="page-main">Back to Main</a>
            <a data-page="page-login">Logout</a>
        </nav>
        <h1>SETTINGS PAGE</h1>
    </div>
</div>