How does Solidity stores mapping in a ehtereum smart contract?

mappingStorage.sol

pragma solidity >=0.4.22 <0.6.0;

contract Mapping {
    
    mapping(string => uint256) store; // stored at slot 0
    
    function getAssembly(string calldata _key) external view returns(uint256 _value) {
        bytes32 dir = keccak256(abi.encodePacked(_key,uint256(0))); // concatenation of key and slot
        assembly{
            _value := sload(dir) // load value
        }
    }
    
    function get(string calldata key) external view returns(uint256) {
        return store[key];
    }
    function set(string calldata key, uint256 _value) external {
        store[key] = _value;
    }
}