Because Beanstalk is a single contract, all of the Facets can share a common state through the AppStorage pattern.
Each Facet should define the AppStorage storage variable in the base contract and define no other internal or public state variables.
The AppStorage struct can be found in the AppStorage.sol file.
When adding new state variables, always add them to the end of the AppStorage struct to ensure consistent mapping of state variables to storage slots. When deprecating an unnecessary state variables, either replace it with a different variable denoting that the state variable is deprecated or leave it as is.
In the EVM, data is accessed 32 bytes at a time (known as a slot). Because of this, in Beanstalk, many state variables are packed within the AppStorage struct to reduce gas costs. For example, say that in function foo, the state of 2 uint256 values is changed. Changing a non-zero value to another non-zero value costs 10000 gas (2 * 5000). If the 2 values could be stored in uint128s instead (meaning the data is in one slot), 2100 gas is shaved off, as the state slot is now warm. (If a slot is touched for the first time in a transaction, it is turned from cold to warm for the rest of that transaction. Warm touches are cheaper than cold touches.)