How is eDNA collection done?

At sea the study of eDNA starts from the simple collection of sea water samples (from 1 to several litres).
The sample is then filtered, by forcing the water-sample through a very fine mesh filter (0.22-0.45 micron).
The filter appears as a small (4.7 cm diameter) disk from which, once in the laboratory, all the DNA obtained from the biological traces trapped in it during the filtration phase is extracted.
Thus, we obtain from each single sample, a mixture of millions of DNA traces, coming from the most diverse creatures, the vast majority of which are invisible organisms, such as bacteria and viruses. But there are also biological traces originating from multicellular organisms of all sizes, from planktonic forms, to fish of various sizes up to giants of the sea, such as dolphins and whales.
How can we find among these millions of sequences the (very rare!) ones belonging to the target species, perhaps an elusive living being, such as the monk seal?
“It’s enough” to use specific assays capable of recognizing, among millions of sequences, those (and ONLY those!) made up of a sequence of nucleotides (the building blocks that make up DNA) that is present only in cells belonging to the target species, indicating its recent passage through those waters.
This is what we call a “species-specific assay”: it behaves in practice like a large magnet capable of “fishing” within a huge tangle of sequences, only that -in our case- of the monk seal, clearly only IF present…