When you buy an airline ticket there are up to 3 separate airlines that can be involved for each leg of the flight. These are common known as :
- The "Marketing Carrier". This is the airline who's flight number is on the ticket/itinerary.
- The "Operating Carrier". This is the airline who's aircraft will actually fly the flight.
- The "Ticketing Carrier". This is the airline that actually sold you the ticket.
(The ticketing carrier will be the same for all legs of the flight on the same ticket. The other two can vary from leg-to-leg)
In many cases, all 3 will be the same - you buy a ticket from airline XX, for flight XX123, which is flown by airline XX themselves.
In your case, all 3 are not the same. UA7938 is marketed as a United Airlines flight (thus the UA flight number), but the operating carrier is ANA. What we can't tell from the information you've provided is who the Ticketing Carrier is - and that's the one that actually matters here!
If you purchased your ticket directly from an Airline website, then you can be fairly sure that's the ticketing carrier. If you purchased it from a travel agency, then it's a little more complex. In general, if all flights on a ticket are for the same marketing airline (eg, all UAxxx flight numbers) then most likely the ticket was issued by that airline.
To be certain, you'll need to check the ticket itself. In your email confirmation (or on the relevant website for where you bought the ticket) you should find a "ticket number" which is generally around 13 numbers long. The first 3 digits in this number show who the Ticketing Carrier is. If this number is 016, then your ticket was issued by United Airlines. If it's 205, ANA. If it's 014, it's Air Canada.
Now, back to your original question. The policies that apply in a situation like this will be the policy of the TICKETING carrier. That carrier might be influenced by the OPERATING or MARKETING carriers policies, but officially it is the ticketing carrier that you've paid for your ticket, and it's them that has your money up until the time that you actually take the flight.
In your case it's most likely that United is the ticket carrier so it would be their policies that apply - but you'll need to check the ticket number to be sure!