If not, is there a technology similar to cookies being used?
If you’re looking for storage for your plugins (and not cookies in network requests), you might want to take a look at
xd-storage-helper, which is an MIT-licensed library I’ve written for exactly this use-case.
It allows storing key-value-pair data in a way that’s similar to
set() etc.), and saves the data in a JSON file in your plugin data directory.
If the user signs into an account in her favorite browser to a site like Instagram or similar, would the plugin be able to recognize that and post to her account? Or does she need to login again through a HTML form in UXP panel or dialog?