Looks like at some point during that iPhone's lifespan, it had good amount of data on it. The data was deleted manually and now it is all lumped into the OTHER category. Your iPhone still shows as if (and it is) its storage space is full.
Why don't you restore that iPhone as NEW and that will remove all that data and set up the iPhone as NEW. You have only 16GB of storage space and you are limited right from the get go.
Please review and follow the instructions in this article by Apple ➔ Restore your iPhone to factory settings - Apple Support.
Axel F.