Is it permissible to hold Friday prayers in a mosque inside a factory that accommodates thousands of workers, knowing that the mosque is not open to the public and that the majority of workers cannot go to external mosques due to the long distance?

You are permitted to establish the Friday prayer in your workplace prayer room if you are unable to perform it in the congregational mosque. This is based on the opinion of the majority of scholars who permit multiple Friday prayers due to long distances, small mosques, fear of discord, and similar reasons. It is not a condition for the validity of the Friday prayer that it be performed in a mosque; rather, it is required that it be performed in a city and its dependencies or in a built area. It is valid to establish it in the desert if it is close to buildings. If the factory is within the town or just outside it, there is no harm in establishing the Friday prayer in its prayer room, even if it is not open to the general public, provided there is a valid excuse such as a long distance or fear of harm.