See how to add a button, the URL of the button will be the URL of the Shopify store, so when someone clicks the button >> it directs to the shopify store.
Please, use this HTML Code and add custom buttons to your products?
<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
</head>
<body>
<form action="https://www.w3docs.com/" method="get" target="_blank">
<button type="submit">Click me</button>
</form>
</body>
</html>
This code will create a button. You will need to do for each product.
Here are the HTML codes to create buttons - https://www.w3docs.com/snippets/html/how-to-create-an-html-button-that-acts-like-a-link.html
You can design the buttons by using different codes. Search online to find some.
That's it.