When you installed purchasing in previous versions of Odoo, it was possible to designate products as stockable, receive them in your inventory, and manage many details. Now, in Odoo 12, the receiving of products is much simpler if you do not have inventory installed.
To receive products, you simply go to the Purchase Order and click Edit. Then, go in and specify the quantity you have received:
And just like that, you have marked the goods in purchasing as you have received.