Cloning
can be done. But, it is often a long, and tedious process. To truly clone, one needs to use the same ingredients. As you say, they can be of foreign origin, domestic (it is not as uncommon as you might think), or made from proprietary concentrates they have custom made (in which case cloning is near impossible).
Often, the goal is made much easier, if you aim for something "as good as," instead of "the same as." This still provides a definable target to focus efforts; while providing more tolerance for variation. Be kind to yourself and set reasonable expectations. Sometimes, you can even find something better.
As you look at other's recipes; try to choose recipes that have lots of reviews and ratings. Many places (especially ELR) has lots of trial, temporary, experimental recipes, that should not be seeing the light of day. Recipes, that offer lots/more feedback, are not guaranteed to be winners. But, can at least give a bit more of an actual idea, as to if they are on the right track.
As
@Frenchfry1942 mentioned; it is not at all uncommon, to have to tweak even "good" recipes to suit one's own palate. And that's okay. That is the beauty of DIY: even though almost no one shares identical palates, each of us can make most recipes "better" for our own use.
And, as a side note, I always record who the author of a recipe is. That way, if I like it, I can search for other recipes by the same author. This can help limit the number of disappointing recipes too.
Since you have already found DIY or Die (Wayne Walker's) website, this might be a good article to read occasionally, when you are feeling overwhelmed:
Your Overthinking It - by Wayne Walker
Nicotine, like any other diluent (PG, VG, water, etc.) can affect the way flavor is perceived. you may need a little more, or a little less, but you should still be able to tell whether you are on the right track. I have never heard anyone complain that nicotine (at any level) outright "ruined" a recipe.
Lots of things can affect how an e-liquid tastes. Hardware choices and setup, coil choices, wicking, and airflow can all have large impacts on how any given juice can taste. Try to minimize these changes while you are exploring new DIY recipes.
I have several different hardware setups, that can range from low sub-ohm DTL, to high ohm MTL. But, I have only one setup (the one I most commonly use) to test and develop my DIY recipes. It is not until I am satisfied with them, that any of my recipes see any of my other hardware. Otherwise, all the extra variables, make DIYing that much more difficult.
Take your time. Be patient with yourself. And, don't get discouraged. You can, and will succeed, if you stick with it! I have faith it you!!
We all started out where you are currently. With the same confusion, apprehensions, and doubts. But, we figured it out... and so can you! And, we are always here to help.