"??" is a NULL coalescing operator. If you see the English meaning of coalescing it says "consolidate together". Coalescing operator returns the first NON-NULL value from a chain. For example below is a simple coalescing code which chains four strings.
So if "str1" is null it will try "str2" , if "str2" is null it will try "str3" and so on until it finds a string with a non-null value.
string final =str1 ?? str2 ?? str3 ?? str4;
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