Do you have to use an exiting primary atlassian product in order to separately user Jira Portfolio? Can you use this product independently?
No. There's a clue in the name "Portfolio for Jira"
Portfolio is an add-on for Jira, it adds some data structures to an existing Jira, but the main data it works with is Jira issues (and projects, and users, and Software stuff if you have that, etc).
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