How to use the iterator in Mathematica

Question

I find that Mathematica has the iterator data structure. We can build some iterators like this:
Constant iterator
GeneralUtilities`ConstantIterator[5]

Key value iterator
GeneralUtilities`KeyValueIterator[<|a -> x, b -> y, c -> z|>]

Range iterator
GeneralUtilities`RangeIterator[9]

We can find all iterator functions with
Names["GeneralUtilities`*Iterator*"]

But I don't know how to process the iterator, as Mathematica has no function like next on Python:
>>> string='FhC'
>>> it=iter(string)
>>> next(it)
'F'
>>> next(it)
'h'

Update
Michael E2's answer clear this problem a lot,But I hope the DelegateIterator, IteratorGraph, JoinMapIterator, SingletonIterator, StreamIterator, TerminatorIterator and ToIterator can be discussed still.

Michael E2 · Answer

Such functions set up a one-time iterator, i.e. a GeneralUtilities`Iterator. Its functionality may be inspected with

? GeneralUtilities`Iterator

and so forth. Except for a ConstantIterator which always returns the same value (forever), they go sequentially through values until they return GeneralUtilities`IteratorExhausted.

The main (if internal) utility is GeneralUtilities`PackageScope`PullIterator:

foo = GeneralUtilities`KeyValueIterator[<|a -> x, b -> y, c -> z|>];

GeneralUtilities`PackageScope`PullIterator@foo
GeneralUtilities`PackageScope`PullIterator@foo
GeneralUtilities`PackageScope`PullIterator@foo
GeneralUtilities`PackageScope`PullIterator@foo
(*
  a -> x
  b -> y
  c -> z
  IteratorExhausted
*)

One can see from inspection with ?... that one can do such things as Map, Scan, Fold, Read, etc., over iterators:

foo = GeneralUtilities`KeyValueIterator[<|a -> x, b -> y, c -> z|>];
Map[f, foo]
(*  {f[a -> x], f[b -> y], f[c -> z]}  *)

Normal converts them to a list:

foo = GeneralUtilities`KeyValueIterator[<|a -> x, b -> y, c -> z|>];
Normal[foo]
(*  {a -> x, b -> y, c -> z}  *)

Other functionality can be inspected with ?.

How to use the iterator in Mathematica

Constant iterator

Key value iterator

Range iterator

Update

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