how can i free up my space I deleted almost everything on my iPhone 7plus but yet it still saying storage full

There are a number of apps that have a Recently Deleted folder, which stores deleted items for 30 days before they are permanently deleted.

For example, with Photos......tap Albums and then look for a Recently Deleted album or folder and delete anything in that storage area.

After you are done, be sure to power off the phone completely, wait a few minutes, then power up the phone again.

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 and iPad - Apple Support

Manage your photo and video storage - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

If you are trying to upgrade your iPhone I do not believe an iPhone 7 plus can be updated beyond iOS 15.8. It doesn't have the hardware needed to run anything higher.