Hi there, I'm the developer of ProShot, I saw my app mentioned so thought I'd chime in
Phone cameras generally have fixed aperture lenses. This is to keep costs down and to keep as few moving parts as possible in the camera housing, decreasing the chance of things breaking if the phone is dropped.
With computational photography its getting easier to get around camera lens / sensor limitations. Apps like ProShot can use disparity maps or LIDAR data to produce natural looking portrait photos from both the tele and wide angle lenses.
When it come to slow shutter photography, ProShot gives you several options:
1) A bulb mode to keep the shutter open. This is great for capturing star trails. And if you leave Live Photo enabled it will also capture a video of the long exposure developing.
2) ProShot's Light Painting mode has two submodes (Light, Water). The Water submode allows you to take photos of running water even in full sunlight. And again, leaving the Live Photo option enabled allows video of the exposure developing to be saved as well.
Hope that helps, and enjoy your new iPhone!