Paying does two jobs at once for you: it lodges a record of the order against your account, and it sets the Premium exam going. The going-live part is normally fast — usually inside an hour, and within six at the far end — and once it is done, the exam sits under My Premium Exams waiting to be started. Nothing has to be downloaded, redeemed, or installed in between.
The one thing worth knowing is that your purchase surfaces in two different corners of the account, answering two different needs: one is the paperwork of the order, the other is the exam you actually came to sit.
The Cloud Exam Order Record on Your Account
Sign in, open My account, and the purchase is listed in Orders. That listing is the paperwork side of things — your history of what was bought and when, and where you return for the confirmation of the transaction. It persists there for you to consult rather than being a one-time notice you have to catch as it passes.
When the Cloud Exam Comes Online
Turning the practice exam on usually finishes well inside an hour of payment, with six hours as the outer limit for a charge that needs checking first; a short gap between paying and seeing the exam is normal, not a fault. When it is live it appears under My Premium Exams, alongside anything else you have unlocked, and you begin from there. If nothing has appeared once that window has passed, reach out to support instead of repurchasing; the payment is already on your account.
So a purchase settles into your account as a stored order under My account and, a short activation later, as a startable cloud certification practice exam under My Premium Exams. Sign back in once it is live and the whole of what you bought is there waiting.