The Fix-Its are not aliens
The main characters only speculate that the Fix-Its are aliens from a small planet. But if they were, they must've been on Earth a long time in order to rebuild objects such as photographs, understand language, and cook human food. They'd also have to adapt to using our electrical grid.Or, they were an experiment or project meant to provide automated repair and maintenance that either got accidentally or deliberately released into the world. If it was deliberately, their intent wasn't just to self-replicate but to aid people like the Rileys.
One more thing: when Mason is examining one of them with a magnifying glass, he sees a lot of open structure next to the Fix-It's eye. Structures and a even a monorail that are clearly not designed to operate in a vacuum.
The Fix-Its are automated probes from another planet
They either are a bunch of robotic probes that are sentient or became sentient.