Designer is where you go for a single image, not a whole deck. The deck is the job of Copilot in PowerPoint; Designer runs as a separate web app at designer.microsoft.com and works from a text prompt, a template or a blank canvas. It can also edit a photo you upload, restyling it or removing its background, and it usually returns several options per request. The result drops into PowerPoint, Word, Outlook, Teams or OneNote without a special export step.
Designer is currently a personal-account tool for Microsoft 365 Personal and Family subscribers. Enterprise accounts are not yet supported, so a Copilot work licence does not give you Designer. There is no brand kit in the current version, so check colours and logo use yourself, and never let a generated image pose as a real product screenshot.