Field names from multiple columns containing record

I've removed [Sub fields] = null. If you want to preserve them - let me know.

Result

let
    Source = #table(null, {{[a=[value=10, description="qqq"], b=[gender="male", age=30], c={"test1", "test2"}, d="Some Text"]}, {[x=1, created by=[Name="Martin", Surname="Green"], identifiers={1,10,100}]}} ),
    Ad_Helper = Table.AddColumn(Source, "Helper", each 
        [ a = Record.FieldNames([Column1]),
          b = List.Transform(a, (x)=> if Record.Field([Column1], x) is record then Table.FromRows({{x, Record.FieldNames(Record.Field([Column1], x))}}, {"Field Names", "Subfields"}) else null),
          c = Table.Combine(List.RemoveNulls(b))
        ][c], type table ),
    ExpandedHelper = Table.ExpandTableColumn(Ad_Helper, "Helper", {"Field Names", "Subfields"}, {"Field Names", "Subfields"}),
    ExpandedSubfields = Table.ExpandListColumn(ExpandedHelper, "Subfields")
in
    ExpandedSubfields

Result

let
    Source = #table(null, {{[a=[value=10, description="qqq"], b=[gender="male", age=30], c={"test1", "test2"}, d="Some Text"]}, {[x=1, created by=[Name="Martin", Surname="Green"], identifiers={1,10,100}]}} ),
    Ad_Helper = Table.AddColumn(Source, "Helper", each 
        [ a = Record.FieldNames([Column1]),
          b = List.Transform(a, (x)=> if Record.Field([Column1], x) is record
                                      then Table.FromRows({{x, Record.FieldNames(Record.Field([Column1], x))}}, {"Field Names", "Subfields"})
                                      else Table.FromRows({{x, null}}, {"Field Names", "Subfields"}) ),
          c = Table.Combine(b)
        ][c], type table ),
    ExpandedHelper = Table.ExpandTableColumn(Ad_Helper, "Helper", {"Field Names", "Subfields"}, {"Field Names", "Subfields"}),
    ExpandedSubfields = Table.ExpandListColumn(ExpandedHelper, "Subfields")
in
    ExpandedSubfields