In Solidity functions, developers are allowed to write assembly blocks containing Yul code. For more information about the Yul programming language, please refer to the yul section <yul_section>.
In an assembly block, you can access solidity local variables freely and modify them as well. Bear in mind, however, that reference types like strings, vectors and structs are memory addresses in yul, so manipulating them can be unsafe unless done correctly. Any assignment to those variables will change the address the reference points to and may cause the program to crash if not managed correctly.
Storage variables cannot be accessed nor assigned directly. You must use the .slot and .offset suffix to use storage variables. Storage variables should be read with the sload and saved with sstore builtins, but they are not implemented yet. Solang does not implement offsets for storage variables, so the .offset suffix will always return zero. Assignments to the offset are only allowed to Solidity local variables that are a reference to the storage.
Dynamic calldata arrays should be accessed with the .offset and .length suffixes. The offset suffix returns the array's memory address. Assignments to .length are not yet implemented.
External functions in Yul can be accessed and modified with the .selector and .address suffixes. The assignment to those values, however, are not yet implemented.