You can specify the core symbol when building. In the main chain's case, EOS is the core symbol. The flag to pass is
-s [CORE_SYMBOL]. You can see this in the Build.sh file.
[CoreSymbolName -s <1-7 characters>]
You can also see the logic for defaulting as SYS in the Build.sh
To create the SYS token, you first need to create the eosio.token account.
cleos create account eosio eosio.token [OWNER_KEY] [ACTIVE_KEY]
This assumes you've already imported the default private chain key to sign as eosio as well as set the eosio.bios contract:
cleos wallet import --private-key 5KQwrPbwdL6PhXujxW37FSSQZ1JiwsST4cqQzDeyXtP79zkvFD3
From there you can set the contract to the eosio.token account:
cleos set contract eosio.token build/contracts/eosio.token -p eosio.token@active
Create the token:
cleos push action eosio.token create '[ "eosio", "1000000000.0000 SYS"]' -p eosio.token@active
cleos push action eosio.token issue '[ "alice", "100.0000 SYS", "memo" ]' -p eosio@active
And transfer it:
cleos push action eosio.token transfer '[ "alice", "bob", "25.0000 SYS", "m" ]' -p alice@active
You can then push the system contract to get more of the attributes of the main chain which are described in more detail here.