qgis - How would I build the syntax inside a request in the case I wanted to multiply 2 variables?

I would like add something to my previous question (here: How to build QgsExpressions in python?)

How would I build the syntax inside this request in the case I wanted field_name to be also a variable pointing to a field name?

And in the case I wanted to multiply two numerical variables inside a request?

expression = QgsExpression('variablepointingtofield * variablepointingtolistitem ')