VFX and editing are usually done in separate programs. VFX is done per-shot, so usually only a couple of seconds or so at a time. Editors don't need all the compositing stuff, but they do need a longer timeline and convenient editing tools. The Adobe video editor is called Premiere, but there's no much point in paying the extra monthly if you have iMovie, since you don't yet know how to use all its advanced controls that speed up the workflow for pros. iMovie'll probably do just fine for getting your feet wet (I assume it does layers and basic colour grading, I haven't used it since PowerBooks were a thing).Thanks! i will look into those programs. So imovie wont work for the mashing up two films idea?
You'll be using the VFX program (the compositor, like Photoshop but for video) to produce the individual pieces, called shots, in all of their component layers, called plates- the backgrounds, digital stuff, live action parts and any effects over the top. Usually you'll arrange them all on top of each other in the compositor before rendering out footage to sequence together in the editor, though sometimes you might add plates in the editor for consistency or something.
There's tons of education on this online to get you up to speed. Check out Film Riot on Youtube for VFX and editing techniques (amongst many other things), and Video Copilot (for After Effects) or BlenderGuru (if you go with Blender). Video Copilot is worth watching just for the ideas alone though. BlenderGuru sticks with 3D stuff so you'll have to hunt down video workflows, but it'll get you up to speed with the interface and nodal compositing. They're just the obvious ones, there are plenty of other people producing tutorials out there that may be more specific to what you're doing. If not, maybe I'll do a Blender-for-video series sometime.
If you're still at the "mashing up two films" stage, this may be a much longer process than you were anticipating, unless you have a friend who's an editor and can guide you through it. The internet speeds up learning, for sure, but be aware that depending on real-life time constraints you may not have anything to show on this project for a couple of years or more. A VFX-heavy scifi piece is a huge mouthful to bite off.