Fill Down based on category

OK, that is in accord with your description.  Try the following code below. 

By following the Applied Steps, you should be able to figure out the algorithm, but, in brief:

• Create a table with all dates and Operators
• Merge with the original table
• Group by Operators
• Fill-Down the Inventory in the sub-group aggregations to cover the nulls
• Expand the Grouped tables
• Sort into desired order
• Replace remaining nulls with 0's

let
    Source = Excel.CurrentWorkbook(){[Name="Table34"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Date", type date}, {"Operator", type text}, {"Inventory", Int64.Type}}),

//Create table with all dates and operators
    #"All Table" = Table.FromRecords({Record.FromList({List.Distinct(#"Changed Type"[Date])},{"Date"})
                  & Record.FromList({List.Distinct(#"Changed Type"[Operator])},{"Operator"})}),
    #"Expanded Date" = Table.ExpandListColumn(#"All Table", "Date"),
    #"Expanded Operator" = Table.ExpandListColumn(#"Expanded Date", "Operator"),

//Merge table with original data
    #"Join Tables" = Table.NestedJoin(#"Expanded Operator", {"Date","Operator"}, #"Changed Type",{"Date","Operator"}, "Join", JoinKind.FullOuter),
    #"Expanded Join" = Table.ExpandTableColumn(#"Join Tables", "Join", {"Inventory"}, {"Inventory"}),
    
    #"Sorted Rows" = Table.Sort(#"Expanded Join",{{"Operator", Order.Ascending}, {"Date", Order.Ascending}}),
    #"Grouped Rows" = Table.Group(#"Sorted Rows", {"Operator"}, {
        {"All", each Table.FillDown(_,{"Inventory"})  , type table [Date=date, Operator=text, Inventory=nullable number]}}),
    
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Operator"}),
    #"Expanded All" = Table.ExpandTableColumn(#"Removed Columns", "All", {"Date", "Operator", "Inventory"}, {"Date", "Operator", "Inventory"}),
    #"Sorted Rows1" = Table.Sort(#"Expanded All",{{"Date", Order.Ascending}, {"Operator", Order.Ascending}}),
    #"Replaced Value" = Table.ReplaceValue(#"Sorted Rows1",null,0,Replacer.ReplaceValue,{"Inventory"})
                    
in
    #"Replaced Value"