How to prevent redirection from non-www to www or vice versa?

Question

So how to prevent the redirection?

On the front-end/public side of a WordPress site, that redirection is done by redirect_canonical() which is hooked on template_redirect, and if you really must disable that non-www to www or www to non-www redirection, you can try the following (add the code to your theme’s functions.php file):

For disabling non-www to www redirection:

add_action( 'parse_request', 'wpse_395638_1' );
function wpse_395638_1( $wp ) {
    // If the current URL doesn't have any path and without the www prefix, e.g.
    // https://example.com or https://example.com/?foo=bar, then we completely
    // disable the canonical redirect by unhooking redirect_canonical().
    if ( empty( $wp->request ) ) {
        $host = parse_url( home_url(), PHP_URL_HOST );

        if ( 'www.' . $_SERVER['SERVER_NAME'] === $host ) {
            remove_action( 'template_redirect', 'redirect_canonical' );
        }
    }
}

// This snippet doesn't disable canonical redirect, but the snippet ensures
// that the redirect URL doesn't use the www prefix, unless the current URL
// uses it.
add_filter( 'redirect_canonical', 'wpse_395638_2', 10, 2 );
function wpse_395638_2( $redirect_url, $requested_url ) {
    $host  = parse_url( $redirect_url, PHP_URL_HOST );
    $host2 = parse_url( $requested_url, PHP_URL_HOST );

    // If the current URL doesn't use www, we remove it from the redirect URL.
    if ( "www.$host2" === $host ) {
        $redirect_url = preg_replace( '#^http(s?)://www.#', 'http$1://', $redirect_url );
    }

    return $redirect_url;
}

For disabling www to non-www redirection:

add_action( 'parse_request', 'wpse_395638_1' );
function wpse_395638_1( $wp ) {
    // If the current URL doesn't have any path and with the www prefix, e.g.
    // https://www.example.com or https://www.example.com/?foo=bar, then we
    // completely disable the canonical redirect by unhooking redirect_canonical().
    if ( empty( $wp->request ) ) {
        $host = parse_url( home_url(), PHP_URL_HOST );

        if ( "www.$host" === $_SERVER['SERVER_NAME'] ) {
            remove_action( 'template_redirect', 'redirect_canonical' );
        }
    }
}

// This snippet doesn't disable canonical redirect, but the snippet ensures
// that the redirect URL uses the www prefix, unless the current URL doesn't
// use it.
add_filter( 'redirect_canonical', 'wpse_395638_2', 10, 2 );
function wpse_395638_2( $redirect_url, $requested_url ) {
    $host  = parse_url( $redirect_url, PHP_URL_HOST );
    $host2 = parse_url( $requested_url, PHP_URL_HOST );

    // If the current URL uses www, we add it back to the redirect URL.
    if ( "www.$host" === $host2 ) {
        $redirect_url = preg_replace( '#^http(s?)://#', 'http$1://www.', $redirect_url );
    }

    return $redirect_url;
}

Additional Note: As I said in the comments, preventing the non-www to www (or vice-versa) redirection could lead to CORS errors due to mismatched origin hostname, e.g. example.com != www.example.com, so just keep that in mind.