qsb | Source code of an online web game Quadra Speed Boosters | Game Engine library

First, inserting to row 0 of the QTableWidget every time will likely be a problem, you may want to check that as well.

Answering your question now, to get the value from the spinbox, simply use qsb->value()

See: https://doc.qt.io/qt-5/qspinbox.html

1. QTableWidget should have defined rowCount and columnCount properties. It can be done either via QTableWidget constructor (https://doc.qt.io/qt-5/qtablewidget.html#QTableWidget-1) or via appropriate methods (setRowCounts and setColumnCounts). If it is done already, that's great.
2. insertRow inserts an empty row into the table at given position. ui->tableWidget->insertRow(1) would insert a new row at position 1 only if you have previously defined rowCount and columnCount (see point 1).
3. It depends what is your idea here - if you would like to have at least 4 columns (please note that we are counting from 0 and the QSpinBox is attempted to be put into third column) and insert new product always at first row, your code with point 1 fullfilled will be work fine. But, if your idea is to add a new row each time new product is added, you should rather call ui->tableWidget->insertRow(tableWidget->rowCount()) and use that value to address appropriate row.

You may also want to have a look here for example how to setup QTableWidget: https://wiki.qt.io/How_to_Use_QTableWidget