I’m going to try to give you a more general answer. I’m sorry if it is too basic, but I’m not sure what information you need.
For every group of living things, scientists called taxonomists have made decisions about what the species are and what characteristics, (often called traits) can be used to tell them apart. I won’t go into how these decisions are made, debated, changed, etc. After these decisions are made, the taxonomists have to write down their evidence, thought process, and so on and publish it so that scientists around the world can look at it and (hopefully) agree on telling those species apart based on those traits.
The key thing to know about this with reference to your question is that the traits that are used to tell species apart can be almost anything. Sometimes it is color pattern, or overall shape, or arrangement of parts, or the shape of a particular part, or the number of hairs on the fourth segment of the rearmost genital, or simply a genetic difference between two populations. And so what we naturalists have to look at to tell the difference between species, and whether that is even possible from photos, depends almost entirely on which group of living things we are looking at. There are two lizard species where I live that can most easily be told apart by the color of their eyes, but people who know them well can also tell by their scales. Also near me there are mushrooms that can only be told apart by extracting DNA, and other mushrooms that can be distinguished by how they smell. For many groups of organisms, you have to look at several different traits simultaneously to know what species you have, and in these cases dichotomous keys are essential.
No one in the world knows, or could know, what traits to look at to tell every species from every other species. There are too many species distinguished by too many different traits. But the iNat community is probably the best place in the world to go to find someone who can help you figure out how to ID whatever species you are currently trying to ID. A discussion on some of the ID methods used by some very knowledgeable identifiers is here: https://forum.inaturalist.org/t/identification-tips-for-ids/29796
There are people here who can ID most birds from a photo of a single feather. We have identifiers who know oaks so well they can tell you which three species hybridized to produce the leaf in question. And so on.
That said, there are still many groups that are vastly under IDed. For example, in my area very few caddisfly observations ever get an identification more exact than caddisfly. So there is definitely the opportunity to become the expert identifier in something if you pick a group that others aren’t focused on, and doing so is really useful.