Excel: Power Query - how to repeat block of code for each row in a table

It appears you want to have a result with two columns date and data. If that is the case, all you need to do is expand the table that results from Step 3.

Merely click the double headed arrow at the top of the column

M Code

let
    Source = Json.Document(File.Contents("C:\Users\ron\Desktop\new 3.json")),
    data = Source[data],
    #"Converted to Table" = Table.FromList(data, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Expanded Column1" = Table.ExpandRecordColumn(#"Converted to Table", "Column1", {"date", "data"}, {"date", "data"})
in
    #"Expanded Column1"

Result

You should also set the data types after you have expanded the columns

Lets start from beginning

let
    Source = Json.Document(...),
    data = Source[data],
    #"ToTable1" = Record.ToTable(Column1)

Now we have List of Records stored in #"ToTable1"[Column1]. (Because table Columns are Lists.)

So we can use List.Transform function to manipulate with each Item of this List.

Syntax is

List.Transform(list as list, transform as function) as list

So we need some transform function which works with each Item of the List separately. Let modify a Code Block from the Original question.

MyFunc = (ColumnRow) =>
let
  #"ToTableX" = Record.ToTable(ColumnRow),
  #"TransposeX" = Table.Transpose(#"ToTableX"),
  #"HeadersX" = Table.PromoteHeaders(#"TransposeX", [PromoteAllScalars=true]),
  #"ChangeTypeX" = Table.TransformColumnTypes(#"HeadersX",{{"date", type date}, {"value", Int64.Type}, type any}})
in

#"ChangeTypeX"

So whole List.Tranform function code looks like

List.Transform(#"ToTable1"[Column1], MyFunc)

This returns a List of Tables which could be used in Table.Combine function.

MyResultTable = Table.Combine(List.Transform(#"ToTable1"[Column1], MyFunc))

Put it all together

let
  Source = Json.Document(...),
  data = Source[data],
  #"ToTable1" = Record.ToTable(Column1)
// End of original code used here
  
  MyList = #"ToTable1"[Column1], //For better readability
  MyFunc = (ColumnRow) =>
  let
    #"ToTableX" = Record.ToTable(ColumnRow),
    #"TransposeX" = Table.Transpose(#"ToTableX"),
    #"HeadersX" = Table.PromoteHeaders(#"TransposeX", [PromoteAllScalars=true]),
    #"ChangeTypeX" = Table.TransformColumnTypes(#"HeadersX",{{"date", type date}, {"value", Int64.Type}, type any}})
  in
  
  #"ChangeTypeX",
  
  MyResultTable = Table.Combine(List.Transform(MyList, MyFunc))
   
  in
  
  MyResultTable