Spanish to English Bus/Financial Finance (general) Missing trader fraud

I think my brain's stopped working because try as I might, I just don't understand a phrase. Three Spanish people have tried to explain the meaning but I'm none the wiser :-(

The context:
Missing trader fraud (also called missing trader intra-community, MTIC, or carousel fraud) is the theft of Value Added Tax (VAT) from a government by organised crime gangs who exploit the way VAT is treated within multi-jurisdictional trading where the movement of goods between jurisdictions is VAT-free.

https://en.wikipedia.org/wiki/Missing_trader_fraud

Here is the fragment where the phrase appears:
O tambien se han detallado muchos casos en que la trucha española (sociedad 3) contacta directamente con el proveedor en origen (la sociedad 1) sin pasar por su propio proveedor teórico (que es la sociedad 2). Otras veces, aunque los contactos se producen de un eslabón a otro según lo esperable (de la 4 con la 3, de la 3 con la 2 y de ésta a la 1), se aprecia claramente que la sociedad 4 tiene información sobre **a quien le compra** la sociedad 3 o incluso la sociedad 2, a qué precios, a qué personas, etc.

'truchas' are 'missing traders'.

I don't understand if company 4 has information about who buys from or sells to company 3.

I would also appreciate some help with 'a qué personas' - does it refer to the people who buy or sell?

Thank you in advance!