Table insert / rows to group

You are right to use x as the variable name. If you still use the each keyword (_ as the parameter name), then the internal [Unit] only represents the value of the [Unit] column of each row in the inserttable table.

let
    Source = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTIEYmOlWB0IzwiITcA8J6icKZxnBFfpBGaB5EE8ZxSVziimOENVQnguUJUInhEKD2GmC1gcJB8bCwA=", BinaryEncoding.Base64), Compression.Deflate)), {"Name", "Item", "Unit"}), {{"Name", type text}, {"Item", type text}, {"Unit", type number}}),
	result = 
		Table.Group(Source, {"Name"}, 
			{{"Count", 
				each
					let 
						gt = List.Sum([Unit]), 
						ttotal = [Item ="Gtotal", Name ="", Unit = gt],
						inserttable = Table.InsertRows(_, Table.RowCount(_), {ttotal})
					in
						// It works, but it's not concise enough 
						// Table.AddColumn(inserttable, "GroupPcent", (x) => x[Unit] / List.Sum([Unit]))
						Table.AddColumn(inserttable, "GroupPcent", (x) => x[Unit] / gt)
			}} 
		)
in
    result

First you should read this part:

https://learn.microsoft.com/en-us/powerquery-m/m-spec-functions#simplified-declarations

I think you should know that each keyword is a syntactical alternative to "(_) =>".

Then you need to understand this part:

https://learn.microsoft.com/en-us/powerquery-m/m-spec-consolidated-grammar#field-access-expressions

When there is no variable (identifier) ​​or record structure before [], an implicit method (that is, looking for a variable named _) is used to find the record (or table column). The official method is called implicit-target-field-selection.

Finally, you need to understand the problem of environments, which allows you to understand nested structures.

https://learn.microsoft.com/en-us/powerquery-m/m-spec-basic-concepts#environments-and-variables