What is the basis for the statement that contributing to a company involved in haram transactions makes the contributor a partner by virtue of stock ownership, and does participation here mean sharing in sin according to the shares, or is it metaphorically in the company?

Whoever owns shares in an institution is a partner in it to the extent of their shareholding. Consequently, they are a partner in sin if the company engages in something forbidden, given that they have entrusted the board of directors or the active partners. This ruling applies to this specific aspect and is not generalized. The prohibition of participating in companies that engage in forbidden acts is derived from the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." For the shareholder is an actor in the company's affairs with their money, even if they are not an actor with their hand or opinion.