Quantum theory applies to everything, observers included, and nothing ever collapses. An observer measuring something branches: each branch sees one definite outcome, and every outcome the equations allow is seen in some branch. All the branches are actual, none more real than the rest.
Reality is a multiverse, as Everett’s theory says, and single-particle interference shows it. Photons sent through slits one at a time stop reaching some places when more slits are open, so something coming through the other slits interferes with them: shadow photons, which are the photons of other universes. Every kind of particle has such counterparts, and they make up a huge number of universes like ours.