Welcome to the forum!
Currently the only way to do something like this is to favorite an observation. But you only get one Favorites list, other lists collect species, not observations (as you already noticed). You can do something sort of similar using a Traditional project (as opposed to a collection project), but observers can decide not to allow their observations into projects.
You can see some discussion on this topic previously here:
https://forum.inaturalist.org/t/separate-like-and-bookmark-buttons/418
https://forum.inaturalist.org/t/split-fave-button-into-two-separate-features/1896
In particular, the recommendation was to use browser bookmarks for lists like this: