Add content globally
In Customizer > WooCommerce > Product page – Components you can find 2 toggle options called “Enable text Before/After “Add to cart” Button” eg: https://d.pr/i/2ecMWf .
If you’ll enable any of them, a text editor will pop. Inside these editors you can pretty much add whatever you want, but most importantly you can add global sections, by their shortcode.
Edit this content per product
Simply edit the product and choose if you want to show/hide or override the text.
Adding buttons that would trigger a modal
Here’s a quick video – https://d.pr/v/ydBnFY . Basically you can create a generic global section, and inside it add button elements that would link to a modal.