I have a complex itinerary from Germany to South America.

As I understand it, airlines will ask me for Covid proofs based on my final destination, not the country I am transiting through (which has more stringent requirements).

But there are 3 scenarios here:

1. My two legs are with one airline. I book with them.
2. I book with multiple airlines using a travel agency.
3. I book with multiple airlines myself separately.

The first happens to be very expensive. The second is risky because travel agencies can be bad.

But what about the third option? If I show the first airline proof of onward travel, will they accept that, and not ask about Covid requirements for the intermediary city?