Which Paris metro station to alight for CDG international airport?

The Charles de Gaulle airport (which locals call “Roissy”) is large. It has three terminals and two suburban train (RER) stations.

As the signs in the trains tell you, as well as the automated announcement when you arrive at the station, alight at “Aéroport Charles-de-Gaulle 1” for terminals 1 and 3, and at “Aéroport Charles-de-Gaulle 2” for terminal 2. Station 1 is actually at terminal 3 and you need to take another short-distance train (“CDGVAL”) to reach terminal 1. Station 2 is in the middle of the gates at terminal 2. Just follow the signs in French and English (and some in other languages).

The separation between terminals is not about domestic vs international flights. There are international flights (both Schengen and non-Schengen) at all terminals (I’m not sure if there are domestic flights at all terminals, it might vary from year to year).

Most if not all airlines only fly from one terminal, and there are signs in the trains that list the airlines and the terminal they fly from. Note that what matters is the operating airline, not the airline that sold your ticket; often airlines that are in an alliance fly from the same terminal but not always.

Your ticket should specify which terminal you fly from. Your boarding pass, if you already have it, definitely specifies which terminal you fly from. If you have Internet access, you can find scheduled flights on the airport website.

If you get off the RER at the wrong station, you can take the CDGVAL train to get to the terminal you want.

What is the most efficient way to get oneself from a downtown Paris Metro station to the Charles De Gaulle International Airport luggage check-ins, passport control, and gates, etc?

Generally RER B is the most convenient unless you want to pay for a taxi. However, you might want to avoid it at peek hours (going into Paris around 8–10 am, and to the airport around 17–19 (5–7 pm)) because it can get overcrowded. A ticket between Roissy Charles-de-Gaulle airport and Paris is valid to any RER station inside the Paris city limits (zone 1) or to any metro station (regardless of zone). (A basic metro ticket or the Navigo Liberté equivalent is not valid on the RER outside Paris.) Also note that at the moment, the line closes early in the evening on weekdays due to construction, and when that happens there might be a replacement bus service (which takes the RER ticket); read and follow the signs.

The main alternative is the Roissybus express bus from Opéra. It’s designed to fit luggage, but can get very crowded sometimes too. One ticket (sold on a machine at the bus stop) just gets you onto that bus. (The former “car Air France” a.k.a. “Bus Direct” service shut off during Covid and is not planned to resume.)

There’s a separate network of night buses if for late night arrivals and early morning departures. There are also some commuter services, but they aren’t really relevant for people traveling from downtown Paris, especially with luggage. See the airport website for more information.

attendant seemed to be insisting that she was on her break and could not answer questions right now (…); she was clearly not interested in helping right now and it was rude of me to keep trying.

Yes, it was rude to keep requiring someone to work from free.

RER B stations at Paris-CDG airport:

Terminals 1 and 3: "Aéroport Charles de Gaulle 1" station
• Terminal 1 is available by the free CDGVAL rail shuttle (journey time: 6 minutes).
• Terminal 3 is available by pedestrian walkway.

Terminal 2: "Aéroport Charles de Gaulle 2 TGV" station
• Terminals 2C, 2D, 2E et 2F are availables by foot and by moving walkway.
• Terminal 2A is available by foot or by free N1 shuttle from RER station, level 5.
• Terminal 2G is available by free N2 shuttle, from Terminal 2F, exit 2.10.

RER B stations in Paris:

• Paris : Paris-Gare du Nord, Châtelet-Les Halles, Saint-Michel-Notre-Dame, Luxembourg, Palais-Royal, Denfert-Rochereau, Cité Universitaire
• Link with Paris-Orly airport by connecting with OrlyVal at station "Antony"

Frequency: Every 10-15 minutes on weekdays

Credit: https://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport/rer-b:

Map from https://parisbytrain.com/charles-de-gaulle-airport-cdg-to-paris-by-train/?amp:

Regarding your comment:

our flight bookings neglected to include a terminal number.

Search the flight number on Google. It’ll typically indicate the terminal number. Or one can look at the airport departure page (https://www.airport-charles-de-gaulle.com/cdg-departures for CDG) to see the terminal number.