Product.product is inherits from product.template, so any filed in product.template will automatically passed to product.product by framework

2- So to custom field in product.product you can inherit product.product or product.template and add your new field

If you need to access a custom field value of product.template, from product.product, you can do it easily.
Just browse the product.product record and use the product_tmpl_id field to get the custom field value from product.template table. i.e, like,

for product in self.env['product.product'].search([]):
     print "custom_field value =', product.product_tmpl_id.custom_field