I'm planning to go on September. so I can purchase a ticket and I go everywhere inside universal studios? or I have to to purchase a ticket for Jurassic and then another ticket for transformer or how does it works?

You only need to purchase a ticket to actually be inside Universal and Islands of Adventure and Volcano Bay. Once you're inside you can go on anything you like by just standing in line for the ride☺

City walk is free (minus parking) but there are two parks which cost money to get into. And whatever is in the park you paid for is free

No. The ticket is near $110 and cover one park and every ride inside. Tickets are per park.

Your ticket purchase includes jurassic park

1 Ticket per park.. that's it unless you buy a hopper and you can go between park (Islands of Adventure & Unuersak studios)