How can I properly spawn zombies on the server?

Excuse me, could you please advise on how to properly spawn zombies on the server? Previously, while I managed to initiate zombie spawning technically, the zombies wouldn’t render as visible entities - they could attack players but remained invisible, and players couldn’t retaliate. My intended design is to create a system where consuming a specific item triggers zombie spawning, and defeating these zombies would grant players rewards. However, I’m encountering persistent difficulties in successfully implementing the zombie spawning mechanics. Any guidance would be greatly appreciated.