params need to have only single item...

Not quite correct.
params is a QStringList. A LIST of QString.
With the << operator you direct the right side to the left (i.e. the string list).

params need to have only single item...

So it's more like "You can add only one after another"
(because QProcess needs every argument as separate string)

As you might already know / have seen from @Axel-Spoerl 's suggestion to use qDebug()

This

params << "-fa";
params << "Monospace";
params << "-fs";
params <<"16";

produces:

• QStringList("-fa")
• QStringList("-fa", "Monospace")
• QStringList("-fa", "Monospace", "-fs")
• QStringList("-fa", "Monospace", "-fs", "16")

Each element is a QString and the next to-be-added item is appended to the end of the list.

Or just paying attention what works...

params need to have only single item...

 params << "-fa";
    params << "Monospace";
    params << "-fs";
    params <<"16";

@AnneRanch
Your issue has nothing to do with multiple parameters, string lists or spaces.

Is there a way to verify the actual contents of

QStringList BEFORE running QProcess start?

Of course. Since you put the parameters into a QStringList variable you can qDebug() << params;. That tells you precisely what the contents are before you pass to QProcess, no more and no less than that.

params << "-e" << "lsusb"; // QProcess starts but terminates

Well yes, because that is what you tell it to do with the -e argument. xterm -e whatever says: start an xterm, run just and only whatever in it and then when that completes exit the xterm. So as soon as your lsusb command finishes, which I guess is microseconds, the xterm process finishes, closes its window, and the QProcess receives finished and exit code from the xterm. Tested with xterm -e ls it is so quick you don't even see the xterm appear and disappear.

You can see this for yourself, nothing to do with Qt or QProcess. In a terminal just run xterm -e lsusb. You should effectively see nothing (because it's so fast), and you get back to the command prompt indicating that the xterm has exited.

If you want to see what is really happening try a long-running command with lots of output: xterm -e find / -print. See how you see all the output scroll, and when it gets to the end the xterm window closes and exits.

I don't know what you intend to achieve via xterm -e lsusb other than than this behaviour. If you perhaps want the xterm to run the lsusb command and then stay open with its output then ask for this, it would require a different command line to xterm using the -hold option:

// from terminal
xterm -hold -e lsusb
// from Qt
params << "-hold" << "-e" << "lsusb";
QP->start("xterm", params);

This will run the lsusb and then hold the xterm window open waiting for you to manually close it. The QProcess will not finish until you do so.

Is there a way to verify the actual contents of
QStringList BEFORE running QProcess start?

qDebug() << params;

   QProcess ftp;   

   QObject::connect (&ftp, &QProcess::readyReadStandardOutput, [&](){
      QByteArray data = ftp.readAllStandardOutput();
      auto coutList = data.split('\n');

      qDebug() << coutList;
   });
   QObject::connect (&ftp, &QProcess::readyReadStandardError, [&](){
      QByteArray data = ftp.readAllStandardError();
      auto coutList = data.split('\n');

      qDebug() << coutList;
   });

   ftp.start("wget", QStringList() << "-P" << url.toString());
   return ftp.waitForFinished();