Liquidity comes from different places depending on the stage of the token:
Pre-bond: DBC - Liquidity is virtual. Only one side (the token) is needed, and the bonding curve simulates liquidity until a threshold of 85 SOL is reached.
Post-bond: DAMM v2 - Liquidity comes from users who provide it by depositing tokens into the pool. These deposits make up the actual liquidity that supports trading.
Feel free to check the Meteora documentation for detailed information!