Booking.com - who is the service provider for flight bookings?

[andannuverthing]

DS has a single booking for a 3-leg flight via Booking.com. The first leg is with Vietnam Airlines and legs 2/3 are with Qatar Airways. Each leg has a separate flight number.

I'm concerned about Booking.com's poor Trustpilot reviews for flight bookings. The Booking.com T&C's are generic for all of their services and say changes/cancellations need to be arranged via the "service provider". In this case, would the service provider be the two different airlines? If there is an intermediate service provider then it isn't stated on the booking, but I've read that Booking.com do partner with Etraveli Group and GoToGate for flight bookings.

I think I may have partially found the answer in a different section of the T&C's which says the booking is directly with the airline, but I still don't know if this will be counted as a single booking or two separate bookings with two different airlines.

Working through it ... there is a single booking reference and we've manage to use that on Qatar Airlines website to find the booking. All 3 legs are included there, so I think (hope) they must be the overall Service Provider, with Vietnam Airlines being just their carrier for the first leg.

If so, that is reassuring.