If the element you are interested in is the only element having an id that starts with #pop_ within the page, you can select it as follows:
[id^=pop_] > div > div > div.inner-content > div > div > div > button
If there are multiple elements having an id that starts with #pop_, then you will need to find out a pattern that can differentiate them.
For example, if the element you want to find always appears as the first element, you can add some additional constraints to the selector, or filter the result after the selection.
const elements = document.querySelectorAll('[id^=pop_] > div > div > div.inner-content > div > div > div > button');
// highlight the selected elements
elements.forEach(el => el.style.background = 'tomato');
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
<div id="pop_1234">
<div>
<div>
<div class="inner-content">
<div>
<div>
<div>
<button>The Element</button>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<div id="not_pop_1234">
<div>
<div>
<div class="inner-content">
<div>
<div>
<div>
<button>Not The Element</button>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</body>
</html>
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