Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Nullable reference types improve the compiler's model; they do not change the CLR representation or sanitize values arriving from outside the annotated code.
Yes, a variable declared as non-nullable string can still receive null at runtime through deserialization, reflection, older assemblies, disabled nullable context, or an incorrect null-forgiving operator. Nullable annotations tell the compiler and callers what the code intends, and flow analysis warns about paths that may violate that intent. I still validate untrusted input at the API or message boundary. After validation, I map it into a domain type whose constructor or factory preserves the non-null invariant.