Read the link i posted. There aren’t any controlled studies because they would be ethically inappropriate. (“There are no controlled studies of the efficacy of parachutes”) but the gist is that the experts who have studied this all believe masks help, based on the available evidence.
Of course we have decades of experience of surgeons wearing masks to protect their patients from infection. There probably aren’t controlled studies there, either, but enough medical professionals believe it that every person in the operating theater always wears a mask. That, of course, has been the belief that masks reduce spread-from, not infection-to. But on a population basis, reducing spread-from makes a huge difference.
The surprisingly low impact of the black lives matter rallies on covid spread, which had been extensively studied, was the first real data in favor of the efficacy of random cloth masks.
There’s one unpublished study where some participants were urged to wear masks and given masks and others weren’t. The authors say they aren’t prepared to release results, yet. Given that people in both groups listened to the same media, and could buy masks, i wonder how different the behavior of the two groups would have been. But it hasn’t been published.
There are several studies looking at rate of spread in counties with and without mask rules, in various places, and those point to benefits from masks.
I’m pretty sure there was an observational study allowing that people who claimed to have worn make were less likely to catch it. If course, those people were generally more cautious in other ways, too. (Attend fewer gatherings, washing hands). That’s a confounding factor in the “country” studies, too.
There’s the animal study with surgical masks places between cages (or not), showing a large benefit. Of course, on real life masks leak around the edges. But there are also small human studies looking at how much masks actually leak, and they show that mostly masks actually do contain a lot of aerosols, as well as larger particles.
So… No one clear killer study. But a lot of evidence supporting the idea that masks work, along with theory supporting it.