Not all Economy classes were created equal. Each economy class differs in the terms offered by the airline. This includes changing and cancellation terms, meals offered, checked bags, carry on availability, seat selection available, priority boarding, eligibility for upgrades, number of loyalty points accumulated, and the ability to use points.
Your flight search results will indicate the seating class for a specific flight option, with icons highlighting whether meals, checked baggage, or carry-on luggage are available. If you cannot find an Economy Premium result for a specific flight, it might be because that airline does not sell Economy Premium at time of booking, only as an upgrade during seat selection.