There is s hearing but I don’t think the accused participates. A judge decides and it’s like a special protective order. In the meantime, your firearms will be confiscated and you will be barred from future purchases. I think you would have to sue to restore your rights. Each state is different. There is a lot of room for abuse.
This also bypasses a lot of due process provisions including being innocent until proven guilty, facing your accusers, presenting a defense, fair and speedy trials, allowance to be judged by your own peers, etc.
Even today, some people bemoan that the mere open carry of a firearm threatens and oppresses them. I foresee a lot of future filings if red flags are passed based on that alone.