Why my phone full memory?

because you have filled it with photos, videos, apps, etc.

Freeing up storage space on your iPhone.

Here are some helpfull 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