Follow this guide for options on connecting Power Apps to Spectrum

Microsoft hides the per app option for purchasing licenses in the Marketplace. Follow this guide to learn how to find it.

Microsoft does not automatically allocate passes once purchased. You need to apply those passes to the environment where your app resides.

Not only is this not necessary, it is not possible. Assign app passes to the environment and Microsoft will automatically allocate those licenses as users access the apps each month.

Microsoft has been renaming the licenses around Power Apps constantly for the past several years. Depending on where you look they could be called different things. Per User is sometimes called Power Apps Premium and Per App is sometimes called App Passes.

There are a few circumstances where licenses are not required. Apps that connect solely to SharePoint for example doesn't require a license. However anything that will connect to Spectrum for either reading or posting to web services will require a premium or per app license.

This document from Microsoft explains all the available options.