I miss this from my android as well

You have two options. One, add a camera widget to your control center (control center is the menu you get when swiping down from the top right of the phone). You can add it by going to settings > control center and adding camera

The other option is to make a shortcut that opens the camera app, and then use the back tap feature (settings > accessibility > touch > scroll down to back tap) and set a double or triple tap to run that shortcut

The iPhone calculator is so so so awful… I would recommend downloading PhotoMath, and if you don’t want to use the camera feature, there is a typing feature.