This is an interesting and challenging question, with broad applications. The standard answer is "read the contract", to which one should reply "show me the contract". It has to be findable from a web page – a contract can't be a secret document that the accepting party cannot find. At the bottom of their main page, you can find a TOS. It states that
This Website is provided solely to assist customers in gathering
travel information, determining the availability of travel—related
goods and services, making legitimate reservations or otherwise
transacting business with travel suppliers, and for no other purposes.
In other words, they disavow any implication that they are also selling you a product. The agreement is with "the customer visiting the Website and/or booking a reservation through us on this Website, or through our customer service agents", which confirms that they are "facilitating" an arrangement (with a "partner" i.e. the airline). The agreement part comes where they say
By accessing or using this Website, booking any reservations for
travel products or services, or contacting our call center agents, you
agree that the Terms of Use then in force shall apply
In case of dispute, there is a section on "working it out informally", which has the bottom line
You and Expedia agree that any and all Claims will be resolved by
binding arbitration, rather than in court, except that you and we may
assert Claims on an individual basis in small claims court if they
qualify. This includes any Claims you assert against us, our
subsidiaries, travel suppliers or any companies offering products or
services through us (which are beneficiaries of this arbitration
agreement).
The section "SUPPLIER RULES AND RESTRICTIONS" which starts
Additional terms and conditions will apply to your reservation and
purchase of travel—related goods and services that you select. Please
read these additional terms and conditions carefully. In particular,
if you have purchased an airfare, please ensure you read the full
terms and conditions of carriage issued by the travel supplier, which
can be found on the supplier’s website. You agree to abide by the
terms and conditions of purchase imposed by any supplier with whom you
elect to deal, including, but not limited to, payment of all amounts
when due and compliance with the supplier's rules and restrictions
regarding availability and use of fares, products, or services.
Airfare is only guaranteed once the purchase has been completed and
the tickets have been issued. Airlines and other travel suppliers may
change their prices without notice. We reserve the right to cancel
your booking if full payment is not received in a timely fashion.
I went through the exercise, and it does offer a highly unintelligible stream of text that states the "rules", though the text is hosted on Expedia and not the carrier's website (this refers not to the contract of carriage, but to the booking-specific contract details that state the flight-specific conditions. The contract of carriage is on the UA webpage).
The next paragraph states
In case of a no-show or cancellation, you may be entitled to a refund
of airport taxes and fees included in the price of the ticket
purchased. Where you are eligible for a refund, you can request such
a refund from Expedia customer service, who will submit your request
to the airline on your behalf.
You will note the conditionality of this information. They do promise to submit your request to the airline – that's part of the contract.
Your agreement with Expedia is murky, but it appears that you have not agreed to acquire a flight from Expedia in exchange for money, rather, you have agreed to allow Expedia to facilitate your contract with the carrier(s), where there is no clear statement of "consideration" in the agreement, but one may assume that what they get is whatever commission they have negotiated in their contract with the carrier.
It is not clear to me that there is such a thing as "Expedia credit". They refer to airline credit, which is certainly a thing. The first thing they say is "If you’ve had to cancel or change your non-refundable flight, most airlines will offer you a credit instead of a refund". I can attest that issues do arise when an airline cancels a flight then issues a credit rather than a refund, however that was a foreign carrier who changed their policy after a year. The named airline has a (current) statement of refund policy. Your relation with the airline is ultimately reducible to their contract of carriage. Refunds are covered under Rule 27. In theory, they should therefore issue a refund, but also read Rule 24, where para D says
In the event of a Force Majeure Event, UA without notice, may cancel,
terminate, divert, postpone, or delay any flight, right of carriage or
reservations (whether or not confirmed) and determine if any departure
or landing should be made, without any liability on the part of UA. UA
may re-accommodate Passengers on another available UA flight or on
another carrier or combination of carriers, or via ground
transportation, or may refund, in its sole discretion, any unused
portions of the Ticket in the form of a travel certificate or travel
credit.
So it depends on the circumstances.
