are you not from the uk, yet you say your ex wanted to get british citizenship from marrying you.
i only ask because a private limited company is a concept familiar to most british people, i would have thought.
many small businesses have this type of ownership, along with sole traders and partnerships.
it is a non-natural person in legal terms, so that the ownership of a business is a separate entity to the individuals running that business.
although liability may be imputed to the commanding mind, according to legislation and the circumstances, usually in criminal matters.
i have not explained it well, sorry.
as to what advantage he hopes gain, who knows, could be lots of things, mostly financial.
but really i think it is not good for you to be investigating your ex's current life like this.
you still sound enmeshed, to be so interested.
it is not healthy. move on. ignore. live your own life.
all the best.