"When a cartographer
generates the trade offers for the explorer maps, it chooses the
location of one of the nearest structures of the respective type (ocean
monument or woodland mansion) in a chunk that has not yet been
generated. This means that it will always select a structure that no
player has visited yet. This is especially useful in SMP for finding
unraided ocean monuments in order to obtain sponge blocks."
In my experience the presence of a structure within already generated chunks (which will then be ignored by the explorer map generation algorithm) is less an issue with Ocean Monuments as they tend to be easier to see (particularly at night) whereas mansions can more easily be hidden by the terrain, particularly if one explores at ground level rather than across the canopy (or by flying) or if the sections of roofed forest in which an example generates is near hilly biomes.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.