fallback rewrites are checked/applied, these are applied before rendering the 404 page and after dynamic routes/all static assets have been checked. If you use fallback: true/'blocking' in getStaticPaths, the fallback rewrites defined in your next.config.js will not be run.
Rewrite parameters
When using parameters in a rewrite the parameters will be passed in the query by default when none of the parameters are used in the destination.
next.config.js module.exports = {
  async rewrites() {
    return [
      {
        source: '/old-about/:path*',
        destination: '/about', // The :path parameter isn't used here so will be automatically passed in the query
      },
    ]
  },
}
If a parameter is used in the destination none of the parameters will be automatically passed in the query.
next.config.js module.exports = {
  async rewrites() {
    return [
      {