Price per unit is a Shopify admin feature that is currently only available for merchants located in France or Germany - you can read more about this feature in Shopify's documents here:
Currently, our Envy theme has 'Price per unit' support installed as standard to allow the 'Price per unit' measurement to be displayed on the following pages:
Cart page & drawer
Collection list page
You can see an example of the functionality here:
The following elements must be enabled/set for the price to show:
A Store location set somewhere in France or Germany
'Show unit price for this product' is enabled for the product within Products.
A price and measurement have been assigned, in the same space as the above.
The product has no variants enabled - you may wish to offer a product with different variants as separated products, to ensure this all appears correctly.