How to select an element from the innerHTML using javascript only

You don't want innerHTML for this, you just want to access the HTMLInputElement within that cell.

You can do it like this, but it's fragile:

cell.firstChild.checked

I'd probably use querySelector to find the first input within the cell:

cell.querySelector("input").checked

Similarly, I probably wouldn't use cellIndex === 0, so I could change the layout without breaking my code. So for instance:

let table = document.getElementById("hosts");
for (let row of table.rows) {
  for (let cell of row.cells) {
    const checkbox = cell.querySelector("input[type=checkbox]");
    if (checkbox && checkbox.checked) {
      // ...this cell has a checked checkbox in it...
    }
}

Side note:

Beware, the HTMLCollection returned by rows and cells is not defined as being iterable, but you're relying on it being iterable by using for-of. Some browsers make it iterable even though it isn't specified, but others don't.

If you're doing this in a page or application (not a library), you can ensure that HTMLCollection is iterable by polyfilling it like this:

if (typeof HTMLCollection !== "undefined" && HTMLCollection.prototype && !HTMLCollection.prototype.forEach) {
    // Yes, there's really no need for `Object.defineProperty` here
    HTMLCollection.prototype.forEach = Array.prototype.forEach;
    if (typeof Symbol !== "undefined" && Symbol.iterator && !HTMLCollection.prototype[Symbol.iterator]) {
        Object.defineProperty(HTMLCollection.prototype, Symbol.iterator, {
            value: Array.prototype[Symbol.itereator],
            writable: true,
            configurable: true
        });
    }
}

See this answer doing that for NodeList (which is defined as iterable) for details.

Alternately, use querySelectorAll on the table and rows instead:

for (let row of table.querySelectorAll("tr")) {
  for (let cell of row.querySelectorAll("td")) {
    // ...
  }
}