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 and Flow themes have '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
Our Capital theme has this in the pipeline to be included also and will be updated here once complete.
You can see an example of the functionality here:
Elements that must be enabled for the price to show are as follows:
Store location in France or Germany
'Show unit price for this product' is enabled for the product
A price and measurement has been assigned
The product has no variants enabled