Magento 2: Override addtocart.phtml file.

Add to cart button in product page coming from Catalog module with product/view/addtocart.phtml template file.
If you want to customization on addtocart template file in Magento 2, You can override addtocart.phtml file in your module by below way,

Create the catalog_product_view.xml file in your module,
Path: app/code/{VendorName}/{PackageName}/view/frontend/layout/catalog_product_view.xml and block contains the addtocart template file.

Now you have to create template file addtocart.phtml in your module at below location,

copy the clone of template from Core Catalog module’s addtocart.phtml file and do your customization.