Getting a value into a column from the same table

MasonMA has provided the easiest solution.
If you want to maintain your original table format and you do not want to use table joins, here is a solution. It is likely overkill for your solution but it explores table grouping, nested lists, dynamic column renaming, and dynamic column type setting. Cheers.

let
    //example data
    Source = 
    Table.FromRows(
        Json.Document(
            Binary.Decompress(
                Binary.FromText(
                    "i45WMlTSUXIEYkMjIOGbWKmXXFymFKuDJGEKJLxK81JxyeQg9BjBZEzQDINLmKNoiQUA", 
                    BinaryEncoding.Base64
                ), 
                Compression.Deflate
            )
        ), 
        let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, Name = _t, Score = _t, Filename = _t]
    ),
    //type setting of initial data
    initial_type_set = 
    Table.TransformColumnTypes(
        Source,
        {
            {"ID", Int64.Type}, {"Name", type text}, {"Score", Int64.Type}, {"Filename", type text}
        }
    ),
    //get distinct list of filenames
    fileList = 
    List.Distinct(initial_type_set[Filename]),
    //create a type setting list that sets month columns to Int64 and the filename column to text
    typeList = 
    List.Combine(
        {
            List.Zip(
                {
                    List.Transform(fileList, each Text.BeforeDelimiter(_, ".csv")), 
                    List.Repeat({Int64.Type}, List.Count(fileList))
                }
            ), 
            {{"Filename", Text.Type}}
        }
    ),
    //group table by ID and Name using all rows as the aggregate
    group_rows = 
    Table.Group(
        initial_type_set, 
        {"ID", "Name"}, 
        {
            {"AllRows", each _, type table [ID=nullable number, Name=nullable text, Score=nullable number, Filename=nullable text]}
        }
    ),
    //add a column to the nested table that contains list of distinct filenames for the ID Name table
    add_filenames = 
    Table.TransformColumns(
        group_rows, 
        {
            {"AllRows", each let names = [Filename] in Table.AddColumn(Table.Pivot(_, List.Distinct(_[Filename]), "Filename", "Score"), "Filename", each names, type list), type table}
        }
    ),
    //expand the nested table, dropping the '.csv' from the filename columns
    expand_nested = 
    Table.ExpandTableColumn(
        add_filenames, 
        "AllRows", 
        List.Combine({fileList, {"Filename"}}), 
        List.Combine({List.Transform(fileList, each Text.BeforeDelimiter(_, ".csv")), {"Filename"}})
    ),
    //expand the filename list column to new rows
    expand_filenames = 
    Table.ExpandListColumn(
        expand_nested, 
        "Filename"
    ),
    //set the column data types
    final_type_set = 
    Table.TransformColumnTypes(
        expand_filenames, 
        typeList
    )
in
    final_type_set