Can I upgrade storage on my iPhone?

You can't physically update the storage available. If you need to increase storage, your only option is to send some of your stuff to iCloud. You will get 5Gb of free storage, after that you pay according to how much you use. The next level is 50Gb which costs $0.99 in the USA, or €0.99 in Rep. Ireland. All rates are monthly.

https://support.apple.com/en-gb/HT201238

No, the only thing you can do is Optimize your iPhone storage https://support.apple.com/en-us/HT201656

No there is no such option.

Nope, sorry. The only thing that can be expanded is iCloud storage.