How to transform wide-format data into the structure shown in 'Result' sheet using Power Query?

Try the following:

let
    // Load the source table
    Source = Excel.CurrentWorkbook(){[Name="Data"]}[Content],

    // Unpivot all date-measure columns
    Unpivoted = Table.UnpivotOtherColumns(Source, {"Product Brand", "Product Name", "Region"}, "Attribute", "Value"),

    // Split the 'Attribute' column into Date and Measure
    SplitColumn = Table.SplitColumn(Unpivoted, "Attribute", Splitter.SplitTextByEachDelimiter({" "}, QuoteStyle.Csv, false), {"DateText", "Measure"}),

    // Convert the date string to actual date format (assuming dd.MM)
    AddedYear = Table.AddColumn(SplitColumn, "Date", each Date.FromText("2025-" & Text.Middle([DateText], 3, 2) & "-" & Text.Start([DateText], 2)), type date),

    // Remove the original DateText column
    RemovedDateText = Table.RemoveColumns(AddedYear, {"DateText"}),

    // Reorder columns for clarity
    Reordered = Table.ReorderColumns(RemovedDateText, {"Product Brand", "Product Name", "Region", "Date", "Measure", "Value"})
in
    Reordered