These are simply treated as their numerical value which means that above 2^32 - 1 they are strings.
10.4.2 Array Exotic Objects https://tc39.es/ecma262/#sec-array-exotic-objects A String property name P is an array index if and only if ToString(ToUint32(P)) equals P and ToUint32(P) is not the same value as 𝔽(2^32 - 1).