I’ll try an analogy to explain better. The firewall is a lock on the door to your house. Vlans are a rule that to go from one room to another, you must go back out the locked door and back in.
So an attacker tries to come in and can’t pick the lock. You are safe.
Another attacker can pick the lock and get into a room. But if they can pick the lock for one room, they can pick the same lock again and get into any other rooms because it’s the same lock protecting every room in the house.





Why tho?