As explained in the answer, almost always Expedia will issue a single ticket across one or more airlines (the technical term for that is "interlining"), which is different from an itinerary made up of actually separately issued tickets.
In the former case, there's always exactly one airline responsible for getting you to your destination in case of a missed connection or itinerary changes; in the latter case, you're often on your own.
What Kiwi.com is doing with their "separate tickets plus connection insurance" model is called "virtual interlining", and it's an interesting alternative in some scenarios (well-connected airports with many alternatives). But I'd still never risk it on an important connection.
I've had a very bad experience with Kiwi.com myself: I booked a Ryanair flight on them without realizing that Ryanair is actively trying to prevent Kiwi from reselling their flights. Kiwi.com apparently works around this by booking tickets on pools of Ryanair retail accounts, to which they don't share the credentials with travelers – making mobile check-in impossible. (And Ryanair at least at the time was charging over 100€ of a "service fee" for a boarding pass print at the airport...)
This is only marginally related to booking separate tickets, but I suppose the larger point is that it's never a great situation to be stuck between the lines of two companies actively hostile towards each other, when you really depend on their cooperation to get to your destination.
"Official" interline agreements are an explicit statement that two or more airlines will make at least some reasonable effort to get you and your luggage to your destination, and will be on the hook for it (under ICAO regulations) if things don't work out.
I used Kiwi.com for this a few weeks ago (combining multiple European carriers, including Ryanair) and it worked rather well.
My first flight was delayed, and it seemed likely I would miss the second leg flight, so they sent me an email with a list of options to reschedule. I'd purchased the 'Premium Protection', so I think I could choose a new option up to €250 (+ hotel if overnight) without paying any extra.
I decided to risk taking the first flight, as the second airport was much bigger so figured there would be better flights from there. Fortunately the flight on the second leg was delayed too, so I didn't need to change anything.