References types are nullable by default and it is possible to assign a null value to string. What is the point of string? then? What difference does it make in this case?
string? and string are the same type. This is true for all reference types and reference types only.
In a nullable context the ? is a hint telling that null is a valid value for this field, variable, parameter or return value. Omitting ? on the other hand means that a value of null is not expected. It allows the compiler to do a static analysis of your code and to issue a warning when it could result in a null-reference-exception. It also allows you to avoid unnecessary null-checks.