How can I get more storage on my phone

You can't, the storage on an iPhone is a fixed size and can't be increased.

You will have to free up space on your phone if you need more room.

Here are some helpful articles that can guide you on how to free up storage space on your phone.

How to check the storage on your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT201656

Manage your photo and video storage

https://support.apple.com/en-us/HT205703

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686

You are unable to add physical storage onto your iPhone, however, you can use iCloud Photo Library in order to save storage. iCloud + plans with 50GB of storage start at a very low rate depending on your location.

iCloud+ plans and pricing – Apple Support

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.

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

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