Purchased I cloud storage not showing up

If you have upgraded iCloud+ plan, once your request for iCloud+ plan has been activated, you should receive an invoice in your email box.

Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB or 6 TB or 12 TB) then you have an iCloud+ plan subscription.

If you have an iCloud+ plan subscription, you will receive a monthly bill from Apple, check your email box.

You pay monthly for iCloud+ plan storage.

See also the following Apple articles:

How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage -> Manage your iCloud storage - Apple Support

Did you get your receipt for the purchase? If so, you can contact Apple Support for assistance if the added storage is not reflected on your phone. Use the support link at the top of the page here.