Power Query, connection between "Transform File" and "Transform Sample File" - excel

Question

When importing files from folder, Power Query automatically generates 4 helper objects for the main query.
Question is : how does the "Transform File" function receive the content from "Transform Sample File" ?
Sample File -> Parameter 1 -> Transform File()
actual Query calls Transform File()
Transform Sample file content magically appears within Transform File() ....
Object called "Sample File"
let
Source = Folder.Files("C:\OneDrive\A\d_LAB\timestamp_cgc_cat"),
Navigation1 = Source{0}[Content]
in
Navigation1
Parameter called "Parameter 1" ( receives "Sample File" as argument)
#"Sample File" meta [IsParameterQuery=true, BinaryIdentifier=#"Sample File", Type="Binary", IsParameterQueryRequired=true]
Function called "Transform File"
let
Source = (Parameter1 as binary) => let
Source = Csv.Document(Parameter1,[Delimiter="|", Columns=5, Encoding=65001, QuoteStyle=QuoteStyle.None]),
A = custom_step_a,
B = custom_step_b,
C = custom_step_c
in
C
in
Source
Query called "Transform Sample File"
Source = Csv.Document(Parameter1,[Delimiter="|", Columns=5, Encoding=65001, QuoteStyle=QuoteStyle.None]),
A = custom_step_a,
B = custom_step_b,
C = custom_step_c
in
C
Actual Query that the average human gets to fumble with
let
Source = Folder.Files("C:\folder_with_csv_files"),
filter_hidden = Table.SelectRows(Source, each [Attributes]?[Hidden]? <> true),
#"Invoke Custom Function1" = Table.AddColumn(filter_hidden, "Transform File", each #"Transform File"([Content])),
X = custom_step_x
Y = custom_step_y
Z = custom_step_z
in
Z
asking out of curiosity ...

Answer 1

If you are asking where you can select the Sample File:
Instead of clicking on the Combine & Transform Data button, click on the Transform Data button. Then, after you click the button on the Content column to combine the files from the folder into a table, you'll get the Combine Files screen. At the top left of that screen, you'll see a drop-down selector where you can choose the file from the folder contents that you're combining, to use as the sample file.
If you want to change the file to be used for the Sample File at a later time:
Click Sample File, under Queries; then click your Filtered Rows step, under Applied Steps; then double-click the corresponding Binary for it in the Content column. This will create an applied step with the path and name of the file that you used as the Sample File, and one or more other applied steps below it. Delete any such additional steps so that your last applied step is the one named with the path and name of the file and you see an icon representation of your file.
By the way... You may find it more useful to "fumble with" the Transform Sample File instead of the one you were thinking, as that is the template, so to speak, for the actions that will be done to every file brought into the query from the folder.