Unable to download apps despite having iCloud storage on my iPhone 11

You have confused iCloud Storage and System Storage, which are two different things. Apps are stored in the System Storage, which is set when you purchased the iPhone and cannot be increased. iCloud Storage on the other hand is used to mainly store backups and sync data across devices. It is by no means to store apps.

Go to Settings > General > iPhone Storage and clear out some data you don’t need such as photos, messages, videos, emails, etc. Or you can decide to purchase a new iPhone, but this time with more adequate storage for your needs. No one else can help clear your data, not us, not Apple, not those apps that claim to do so. Good luck.

If you have lots of photos and videos, enabling iCloud Photos and the Optimize iPhone Storage option could let your iPhone free up some device storage.

That gives the iPhone permission to store reduced-quality, space-saving copies of some of your photos. The full-size copies remain available in iCloud. (iCloud Photos is not an archival service, and manually deleting photos from your phone will cause the iCloud copies to be deleted, too.)

Zachyy is completely correct. You can buy all the iCloud Storage you want, but it can NEVER add more memory to your iPhone. Once purchased, the memory on an iPhone is fixed for life.

This link from Apple might help you understand it more --> What's the difference between device storage and iCloud storage? - Apple Support