Lewie, if you want to fly a single mission, you select a pilot in "Pilot Dossiers" in the start screen.
then you click on "Quick Combat" also in the start screen.
You now get a menue, where you can choose aircraft, enemy craft, numbers of pilots and their qualities.
If you want to start a Campaign (which is the most immersive in OFF), you select a pilot and click on "Campaign" then.
Where and when the Campaign begins, is sorted, when you create the pilot.
Creating one, you can select time and place and squadron etc.
Then there is a more detailed way to fly "Quick Combat".
In "Workshops" you find "CFS3 QC" at the bottom.
If you click that, you get a menue, where you can select more detail for creating a single mission.
You can even select from a wide variety of weather.