How do I access more storage that I have paid for?

Storage that you pay for is iCloud storage.

Click on this link to discover how to "Upgrade your iCloud storage plan" --> https://support.apple.com/HT201318

If you need more iCloud storage than that to which you currently have access you can subscribe to more.

iCloud storage does not add storage onto your device. It adds storage into Apple's cloud storage system, here you can save backups and photos. There is no way to convert this to device storage.

To view and manage your iCloud storage go here

For example, you could use this to save storage on your phone, by keeping low quality/resolution photos on your phone but keep the high-quality originals in iCloud.

Take a look at this help article for more info Set up and use iCloud Photos – Apple Support