Does iMovie alter original source video?

1. The changes you make are stored within the app itself (so to speak). You don't directly have access to those files, but you will notice that the size of the iMovie app balloons once you start editing a project.

In addition, video that you capture or import directly in iMovie is stored there (as opposed to your camera roll). You can see this content when in a project by tapping the media browser and choosing iMovie Media (this is also where you can delete, share, or move the content)

2. Currently, you cannot take an iOS project and edit it on your Mac. 😟 (you used to be able to, hopefully they add this feature back!) You can archive a project, or share it for editing on other iOS devices (or as a copy on your Mac, allowing you to free up the space on your device). You do this via the share option on the project screen, then choose share to iTunes, iCloud or using Airdrop.

iMovie iOS is non-destructive. It does not alter the original clip.

However, it's very important to note that it does not copy the clip in to iMovie either. It links to the original clip in your Photos app. In other words, if you assume that you now have two copies and decide to delete the original clip in Photos, iMovie will give you an error indicating the original clip cannot be found and your movie is now broken.

(You do have an option save/archive your project to iTunes or iCloud at which point iMovie WILL copy the source files into your archived project. Only then can you safely delete the original)