SOLVED!
Thanks. Although that was not the solution, i got it.

void cal::set_text_cursor()
{
QTextCursor cursor1= ui->textbox->textCursor();
cursor1.atEnd();
ui->textbox->setTextCursor(cursor1);
}

and then implement that into every digit/operator (position where it has to be set to the End).

another solution is to use (puts the cursor to the end):

ui->textbox->clear();
ui->textbox->insertPlainText(contenttextbox+var);

instead of (puts the cursor to the beginning):

ui->textbox->setPlainText(contenttextbox+var);

Thanks 4 helping!