Update your iPhone to current; to iOS 13.7. That can resolve an issue with “other” storage.
Clear Other storage on your iPhone or iPad - Apple Support
There is no means to expand the internal storage on an iPhone; no hardware upgrades. If the internal storage capacity of your iPhone is insufficient for your current needs, you will either need to delete or to offload some of the existing contents, or replace your iPhone with a model with more internal storage; with a more capacious model.
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
iCloud storage can be offloaded or deleted, or additional storage can be acquired. This if you’re not already on the top iCloud storage tier. iCloud storage can be used to offload some of the contents of an iPhone, and for iPhone backups. Details on managing iCloud storage and buying more iCloud storage:
Manage your iCloud storage - Apple Support