Split/copy row (Year, Quarter, Month)

let
    y2q2m = (prod as text, year as text)=>
    let 
    
    q2m= List.Transform({1..4},    (q)=> [qProd= prod&"-Q"&Text.From(q)&"-"&year ,mProd= List.Transform({1..3}, each prod&"-"&Date.MonthName(#date(2020,_+(q-1)*3,1),"en-US")&"-"&year)]) 
   
    in Table.FromColumns({{year},{q2m}},{"yProd","qmProd"})
in
    y2q2m

invoking the function

let
    Source = y2q2m("P", "2020"),
    #"Expanded qmProd" = Table.ExpandListColumn(Source, "qmProd"),
    #"Expanded qmProd1" = Table.ExpandRecordColumn(#"Expanded qmProd", "qmProd", {"qProd", "mProd"}, {"qProd", "mProd"}),
    #"Expanded mProd" = Table.ExpandListColumn(#"Expanded qmProd1", "mProd")
in
    #"Expanded mProd"

get the table

Anonymous - So what are you starting with, a list of quarterly periods and a list of monthly periods? What do those base tables look like? Are they related?

Not really enough information to go on, please first check if your issue is a common issue listed here: https://community.powerbi.com/t5/Community-Blog/Before-You-Post-Read-This/ba-p/1116882

Also, please see this post regarding How to Get Your Question Answered Quickly: https://community.powerbi.com/t5/Community-Blog/How-to-Get-Your-Question-Answered-Quickly/ba-p/38490

The most important parts are:
1. Sample data as text, use the table tool in the editing bar
2. Expected output from sample data
3. Explanation in words of how to get from 1. to 2.

Gurus have provided preceding solutions above. Mine is for practice of List.Generate only

let
    Source = Table.FromColumns({{2020..2022}, {"A".."C"}}, {"Year", "Product"}),
    #"Added Custom" = Table.AddColumn(
        Source, "Custom",
        (EachRow as record) => List.Skip(
            List.Generate(
                () => 0, each _ <= 12, each _ + 1,
                each
                [y = EachRow[Year],
                q = Date.QuarterOfYear(#date(y, _, 1)),
                m = Date.ToText(#date(y, _, 1), "yyyy-MMM"),
                str = EachRow[Product] & "-" & Text.From(y) & "-Q" & Text.From(q) & ";" & EachRow[Product] & "-" & "-" & m][str]
            ),
            1
        )
    ),
    #"Split Column by Delimiter" = Table.SplitColumn(Table.ExpandListColumn(#"Added Custom", "Custom"), "Custom", Splitter.SplitTextByDelimiter(";", QuoteStyle.Csv), {"YearQrt", "YearMonth"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"YearQrt", type text}, {"YearMonth", type text}})
in
    #"Changed Type"

...