Trying to connect to a cloud database (DB System) on Oracle Cloud is a pretty straight forward thing to do.
First you need to know the IP address of your database. You will find this IP when you are on the overview page of all the databases from Menu → Bare Metal, VM, and Exadata.
The second step is that you will need to put together the Service Name of your database from the details page of your Db System. You need to retain these 2 settings: Database Unique Name and Host Domain Name :
Copy those 2 details in a Notepad.
The second step is to open the 1521 port on your Oracle Cloud account so SQL Developer can connect. You will do this by reading my other story here : https://email@example.com/oracle-database-cloud-open-ports-on-oci-1af24f4eb9f2
Then the final step is to connect with SQL Developer. The settings needed are these:
Connection Name : Write any name for this new connection
Username: SYS (you can use other username after you create them inside the database)
Connection Type: Basic
Role : SYSDBA (when you use SYS account)
Hostname: Public IP of the database
Port : 1521 (unless you changed it to something else when creating the database)
Service Name : Database-Unique-Name.Host-Domain-Name so use the two you copied from the previous step.
If you connect to a PDB for 12c and 18c databases, you need to use the PDBname.host-domain-name, so something like this for my database: PDB01.sub105.mynetwork.oraclevcn.com
Hope it helps!