If you want to create a sitemap dynamically you could create a sitemap.ts file and place your logic there.

Example:

import { API_URL, SITE_URL } from "@/utils/constants";
import { MetadataRoute } from "next";

export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
const data = await fetch(${API_URL}/user/usernames);
let usernames = ["dummyuser"];
if (data.ok) usernames = await data.json();

return usernames.map((username: string) => {
return {
url: ${SITE_URL}/${username},
changeFrequency: "weekly",
};
});
}

should i place in the root folder?

inside app folder

is your issue resolved?, if yes please mark the response as answer.

Thanks

how to mart solution?

see here