It is possible to pre-send ethers to smart contracts because their address is computed in a deterministic way. In fact, a smart contract developer could get the address of the future contract by hashing his own account address (used to deploy the contract) plus the associated nonce (used in the deployment transaction); and next send some ethers to the obtained address. Then the new deployed contract will have a non zero ether balance.
State changes come from the transactions in Ethereum. If one sends some ethers to a valid address and the transaction is included in a block, nodes will reflect the changes and update their different tries. It is possible to send ethers to a non deployed contract or an address nobody owns the private key (in this case the ethers will be locked forever).
In Ethereum, state changes come from the transactions, and only the transactions. For exemple when I create a new address, it is not recorded in the blockchain. Then, if someone sends me some ethers, nodes will mine the transaction and see my address is not present in their world state trie. Hence they will store the new account with its different relevant information (nonce, balance, etc). The same principle applies to "unregistered" smart contract addresses.
To support my point, here is a very good explanation I found on GitHub (https://github.com/tpmccallum/ethereum_database_research_and_testing/blob/master/leveldb/leveldb.md) :
Interestingly, accounts in Ethereum are only added to the state trie
once a transaction has taken place (in relation to that specific
account). For example, just creating a new account using "geth account
new" will not include that account in the state trie; even after many
blocks have been mined.
However, if a successful transaction (one which is included in a mined
block) is recorded against that account, then that account will then
appear in the state trie. This is clever logic which protects against
a malicious attacker continously create new accounts (in the command
line at no cost to the attacker). Such an activity would bloat the