Installing QGIS Server - qgis_mapserv.fcgi returns error

I am installing QGIS Server on Ubuntu 20.04 (focal)

Docs indicate one should run usr/lib/cgi-bin/qgis_mapserv.fcgi and expect:

QFSFileEngine::open: No file name specified
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver JP2ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver JP2ECW to unload from GDAL_SKIP environment variable.
Content-Length: 206
Content-Type: text/xml; charset=utf-8

<ServiceExceptionReport version="1.3.0" xmlns="http://www.opengis.net/ogc">
 <ServiceException code="Service configuration error">Service unknown or unsupported</ServiceException>
</ServiceExceptionReport>

But I get:

Application path not initialized
Application path not initialized
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver JP2ECW to unload from GDAL_SKIP environment variable.
"Loading native module /usr/lib/qgis/server/libdummy.so"
"Loading native module /usr/lib/qgis/server/liblandingpage.so"
"Loading native module /usr/lib/qgis/server/libwcs.so"
"Loading native module /usr/lib/qgis/server/libwfs.so"
"Loading native module /usr/lib/qgis/server/libwfs3.so"
"Loading native module /usr/lib/qgis/server/libwms.so"
"Loading native module /usr/lib/qgis/server/libwmts.so"
QFSFileEngine::open: No file name specified
Content-Length: 102
Content-Type: application/json
Server:  QGIS FCGI server - QGIS version 3.16.1-Hannover
Status:  400

[{"code":"Bad request error","description":"Requested URI does not match any registered API handler"}]

The commands I have entered into a fresh Ubuntu 20.04 image are:

sudo dpkg --remove-architecture i386
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-key F7E06F06199EF2F2
gpg --export --armor  F7E06F06199EF2F2  | sudo apt-key add -
sudo add-apt-repository "deb https://qgis.org/debian focal main"
sudo apt update
sudo apt-get install python-qgis qgis-server libapache2-mod-fcgid
sudo apt autoremove

 // Starts FCGI loop                                                                                                                                                                                           
 while ( fcgi_accept() >= 0 )
  {
  QgsFcgiServerRequest  request;
  QgsFcgiServerResponse response( request.method() );
  if ( ! request.hasError() )
  {
    server.handleRequest( request, response );
  }
  else
  {
    response.sendError( 400, "Bad request" );
  }
}
app.exitQgis();
return 0;