When you say “plug-ins, ” if you mean the Photoshop plug-in modules that have been around since the 1990s, the answer is simple: Plug-ins installed with Photoshop should be included and no extra cost, while plug-ins not installed with Photoshop may come from other companies, and those companies determine whether their plug-in is a one-time payment, subscription, or free.
But reading your entire title, it sounds like you’re really asking about the new third-party generative AI models? I’m only asking because it sounds that way but “generative AI” wasn’t mentioned.
If third-party generative AI models are what you’re wondering about, see if the web page linked below helps. It talks about many third-party generative AI models, which Adobe applications use them, their costs in generative credits, and a lot of Q&A at the end.
Partner models in Adobe products
https://helpx.adobe.com/firefly/web/get-started/learn-the-basics/non-adobe-models-in-adobe-products.html