How to add 'add to cart' button in custom content type using Commerce 2?
Follow below steps to show Add to cart button into any content type.


Add field with reference as Product and select list of product bundles you want to show in content type.
And in field settings select reference as shown in below.

Go to content type manage display and select as "Rendered Entity" as shown below.

Go to the product type which you selected in content type and click on manage display.
See for variations and select as "Add to cart form" as shown in below.



And that's it after adding node with product you will be able to see the Add to cart button into node view like below.