How do I merge two files?

[anonymous3]

I know, the title seems ultimately super ignorant, but honestly, I am not trolling.

Could anyone explain me, how the actual merge workflow works?

I mean, I carefully read the documentation page: http://tour.winmerge.org/#d0e300

And I am still confused. So, I am opening two files, A and B, and see them side by side.

I see where one differs from another. Now I need to make a third file looking like a mixture of A and B.
But there are only 2 buffers in WinMerge! How do I get the actual merge results? Of course, I could just copy all the changes to A or to B, but I don't want to damage the source files.

Ideally (in the future), I'd like to get two source trees as inputs and get a third tree as an output, but for now I'd be happy to obtain that with just two files.