How to access the majority of storage on my iphone 6 plus?

I keep unloading all the pictures I take but for some reason they are all kept in a copied form even if I format the phone.

So every time I unload all the stuff on the phone I have a little less space to use. Until this last time it only let me save about 50 images. Then after I unloaded the phone, it still has less than a gig free. And I keep getting that annoying advertisment about adding more space for a fee. F that. I just want the space I should have.

How do I navigate beyond all this nonsense and actually delete everything since something is preventing me from doing that via formatting?

You should reset your phone to get rid of leftover garbage files. It won't erase anything else. Hold down the On/Off button and the Home button at the same time for about 6 seconds. The phone will then reset which will take about a minute.

Sounds like you may be saving some/all of your photos to iCloud which only has 5GB that is free. When you get close to the limit, you get the reminders about buying extra iCloud storage. Did you intend to use iCloud?