Having an instance up and running is one thing, and perhaps just a single Linux ‘sandbox’ to run some code is all that is needed, however the real power of computers, virtual or otherwise, is in connectivity, and that means networks. The instances in the Rapid Access Cloud can be connected to and accessed in a variety of ways, permitting users to create an environment with multiple instances networked together in the same way a bare-metal environment can be built, but in this case with virtual machines providing the routing, switching and other network functions along with the expected servers running applications on top of operating systems like Linux and Windows. In the RAC web client there is an option to access your instance via a web console. Windows instances can be accessed with the console. Linux instances will prompt for a password and can not be accessed via the console without first logging into the instance (SSH) to configure a password. For this reason it is recommended to only use SSH and a private key to access a Linux instances. Create a keypair before creating a Linux instance then when creating a Linux instance, assign the key for access. See this page for instructions on how to create a key pair.
The commands to Access access a Linux instance with a private key (myKey.pem) depends on the operating system you are connecting from and the SSH console you are using.
For example in macOS, using the default console and connecting to an Ubuntu OS, the command would be similar to:
ssh -i /path/to/mykey.pem ubuntu@2605:fd00:4:1000:aaaa:ffff:bbbb:cccc