How to get more iPhone storage

KiltedTim is right. You cannot increase iPhone storage. But you can optimize the storage on your current iPhone. You can do this and possibly free up space, by deleting apps, deleting old photos you no longer need, old emails & messages. You can read more about it here: How to check the storage on your iPhone and iPad - Apple Support

Getting more iPhone storage is not possible. You will have to make do with what you have or upgrade device with larger storage. You can use iCloud Photos in combination with its Optimized Storage setting to reduce the amount of photos stored on your device and it will only download high-resolution photos when needed. Other than that, you would need to remove apps that you do not use that often and make use of iCloud Drive to store large data files in the cloud instead of on your device. Another area you can check for wasted storage space on your device is deleting old message conversations that contain photos and videos that are no longer needed.

It is not possible to increase the amount of local storage on an iphone.