How to check the current settings of SSH encryption method and key size? When these keys will be used?

Issue

• There are lots of encryption method such as aes128-ctr, aes192-ctr, aes256-ctr, arcfour256, arcfour128, aes128-cbc, 3des-cbc, blowfish-cbc, cast128-cbc, aes192-cbc, aes256-cbc, arcfour. If we do not define the cipher in /etc/ssh/sshd_config, which specific method will be used?
• There are asymmetric key (RSA) and symmetric key (i.e. AES) for SSH. We are very confusing which phase will use various types of key?
• Can you make a brief description on how SSH works?
• We want to change the encryption bits on his server from the default of 1024 to either 2048/4096 bits for better security.