They don’t.
You randomly position yourself somewhere in the tiny sliver of a sub-band, and TX. An important bit of WSPR operation is to only transmit in 10-20% of the windows, not all of them. I guess it is a form of poor man’s TDMA.
Another trick, also used by FT8 SWL mode: you decode a batch of signals, you regenerate their audio and subtract that from your RXed audio. This is called a pass. Repeat for a few passes, decode after each.
Hidden signals discovered like this are not perfect and rely heavily on FEC, especially if something else masks them, but hey do show up.
Thx for the answer, also very interesting, never heard about that FT8 audio subtraction method before but makes total sense :D
My assumption would be it has some form of dynamic frequency selection algorithm so it sits there and listens for a bit to find an empty channel and uses it for broadcast.
