This is a very simple feature which shows a button that has the ability to instantly send the visitor to the Checkout, to purchase a product.
The settings can be located at Customizer > WooCommerce > Product page > Components in Summary > Buy now eg: https://d.pr/i/YvMYTp .
A quick demo can be viewed in this video .
If you want to control on which types of products to show the button, here’s an easy php code snippet you could use https://d.pr/n/R4X8dC .