Struct keys are atoms, so your assumption doesn’t hold.
Maps smaller 30ish entries are (internally) implemented as a tuple with a list of keys and a list of values, and some tree for bigger ones. Both variants allow you to share parts of the unchanged data.
So if you have a struct of size X, and “change” only one keys value, then you will have to copy the structs keys and value “pointers” and set the one changed value pointer to point at the new value. Allowing you to reuse all the actual keys and unchanged values.
