Everyone - I have an actual solution to this problem.
The problem, as a few repliers seem to have missed, is splitting a string into an array of characters. So, for instance, if we had the string "ABCD", we want the array <code?["A", "B", "C", "D"]. We're not trying to split on spaces, or commas, or any other delimiter - we want the individual characters in the string.
The solution is to use the Select action with the range() and substring() functions. If you didn't know, Select is kind of like a map function - it produces a given output for each element of the input. range() creates an array from a given range of numbers. substring() creates new strings from a selected range of a given string. Together, we have all the ingredients we need.
The flow overall.
The secret sauce here is the little icon in the bottom right of the Select action - that switches it from the default behaviour mapping an array to an array of dictionaries, to the far more useful behaviour of mapping an array to an array of… well, anything. The icon looks like a little T in a box.
Here's the expressions we want, assuming the string is in a variable called StringToSplit:
From:
range(0, length(variables('StringToSplit')))
Map:
substring(variables('StringToSplit'), item(), 1)
Let's say we have the string "Hello, world!". The first expression creates an array of integers, starting at 0, up to the length of the string (13 characters). So we get the input array [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. The map expression is then applied to each of these. The first iteration, we have the input item 0, so the map expression becomes substring('Hello, world!', 0, 1) - in other words, give us a substring from the start of the string, that's one character long. So, we get 'H'. The second iteration, the map expression becomes substring('Hello, world!', 1, 1) - give us a substring starting one character from the start, one character long. That's 'e'. And so on until we get to the last element, which becomes substring('Hello, world!', 12, 1) - skipping the first twelve characters and giving us the next one, which is '!'.
The result: ["H", "e", "l", "l", "o", ",", " ", "w", "o", "r", "l", "d", "!"]
I created a gist with a short demo, in Logic Apps and Power Automate flavours. You can check it out here: https://gist.github.com/gormster/acbdc30bffab6bd54e45e02605a672d5
PS: Why can't we use <code> tags in this forum? You have CSS for it and everything, but it gets stripped out when I click Post. Seems crazy to me.