Trimming DOES NOT make the hair grow thicker from the scalp.
BUT
It does let hair look and feel thicker if the ends are straggly and thin, and those ends are trimmed off...
If your hair is thin from the roots to tips, then trimming isn't going to thicken it up.
If your hair is pretty thick from the roots but tapers into thin air, then your hair may feel and look thicker with a trim (and shorter ofcourse)