content-box.css

.content-box {
  --content-box-border-color: #ccc;
  border: 1px solid;
  border-color: var(--content-box-border-color);
  border-radius: .4rem;
}

.content-row-headline {
  display: block;
  padding: 1.5rem;
}

.content-row-description {
  display: block;
  padding: 1.5rem;
  background: #fafafa;
  border-top: 1px solid;
  border-radius: 0 0 .4rem .4rem;
  border-color: var(--content-box-border-color);
  color: #757575;
}

.content-row-description.hidden {
  display: none;
}

.content-row + .content-row {
  border-top: 1px solid;
  border-color: var(--content-box-border-color);
}

.content-box.content-box--list > .content-row {
  display: grid;
  margin: 0 1rem;
}

.content-box.content-box--list .content-row-headline {
  padding: 1.5rem 0;
}

content-box.html

<div class="content-box">
  <div class="content-row">
    <label class="content-row-headline">
      <%= radio_button(f, :test1, true, checked: true) %> True
    </label>
    <div class="content-row-description">
      Description True
    </div>
  </div>

  <div class="content-row">
    <label class="content-row-headline">
      <%= radio_button(f, :test1, false) %> False
    </label>
    <div class="content-row-description">
       Description False
    </div>
  </div>
</div>

content-box.js

function traverse(callback) {
  document.querySelectorAll(".content-row").forEach(node => {
    const input = node.querySelector('input:not([type="hidden"])');

    if (input) {
      const description = node.querySelector(".content-row-description");

      if (description) {
        callback({
          input,
          description
        });
      }
    }
  });
}

function changeState({ input, description }) {
  if (!input.checked) {
    description.classList.add("hidden");
  }

  if (input.checked) {
    description.classList.remove("hidden");
  }
}

window.addEventListener("change", e => {
  if (e.target.parentElement.className === "content-row-headline") {
    traverse(changeState);
  }
});

// FIX for Chrome Version 80.0.3987.149 (Official Build) (64-bit)
// Wait for chrome autocomplete
setTimeout(() => {
  traverse(changeState);
}, 0);