Generally speaking, when defining a structure and defining its members, the structure has not been completed. It is found that the compiler will make an error if it refers to an undefined structure (that is, itself).

Is there something hidden behind "based on this structural type"? I don't understand here. Can other structures be understood based on this type of structure?

Then this "other structure" must have been constructed. If it is referenced in the original parent structure, the compiler will not make mistakes, but I think there will be some problems in using it.