you need to learn a lot before you can make perfume. i you have a question on a certain subject, use this way to search: enter site:www.basenotes.net/ query
into the google search box, query
being some keywords to search for, like 'aroma chemicals'.
to answer one of your questions, aroma chemicals are the molecules we can smell, in particular the ones that are useful for perfumes and artificial flavouring in drinks and foodstuff (most of it is the same, for example raspberry candy can use largely the same molecules as raspberry lemonade and raspberry perfume).
essential oils and absolutes (please look up this current thread
about the differences) consist more or less entirely of these. there can be hundreds of them in a single oil.
some of them are famous, like vanillin (think vanilla sugar, but also present in large doses in a vanilla bean; or caron pour un homme) but most of them aren't and have strange names like para-cresyl phenyl acetate (which is also know as benzene acetic acid 4-methylphenyl ester and a host of other names, but everybody seems to use the former).
there must be thousands of these molecules. i do not really know how many. new ones are constantly being developed. people probably still are discovering some natural ones, too.
in general, small molecules evaporate fast and these are topnotes, the big ones go slower, all the way up to the musks, and beyond that we can't smell them no longer and thus they aren't considered aroma chemicals any more.
aroma chemicals can come from a natural source, they can even be isolated from their source, or they can be manufactured synthetically. the result is in theory the same (impurities can make a difference though!) some aroma chemicals are not known to exist in nature, they where invented by men in white coats.
i think this should do for now as a basic introduction on this topic. there is a lot
more to learn about them, and you will find a plenty to read when you browse and search this forum. use it to your advantage. good luck!