OpenEXR is a digital file format created by Industrial Light & Magic to serve as an intermediate data file which would exactly capture the data produced by a render step. No encoding, no loss, no rounding.
MultiLayer OpenEXR is an extension to that idea which was pioneered by The Blender Foundation(!), then quickly adopted by the industry because it puts many “named layers” of data into a single file.
EXR files are the best choice for all of the files that you produce during your render job, including the final. Then, you use that as a data-source for generating the “deliverable” video file(s) that the audience will actually see. In this way, you have never “lost” nor “encoded” anything … except as a necessary step in producing a “video file” which of course is “the end of the line.”
The compositor, like most other things in Blender by now, is based on “nodes,” and it’s actually relatively easy to get familiar with.