Alcohol, water and glycerol (glycerin) are all polar. Whatever you are looking to extract from the plant/fungi has to be soluable in that, otherwise if it is a nonpolar compound it will just kind of "clump" and form a layer eventually, if you manage to get some in there. For nonpolar compounds, what nonpolar solvent do you guys use? I've always just used oils, but being in a lab lately and doing extractions of a solution with a non polar solvent got me thinking... do you first make some kind of suspension and then do the extraction..... ? or will it be extracted anyways....?
The basic layman explanation has already been given. You have to be familiar w/the specifics of the actual plant/herb/flower/bark, etc. and to determine the best way to extract the materials/constituents you want from said plant/herb/bark, etc. Usually this is done by means of referencing research material (certain books, journals, i.e.) and/or personal memory of learned references (similar to memorizing the periodic table of elements. i.e.). I already gave a brief explanation already when I mentioned knowing what percentage of alcohol is preferrable for extraction (it's not always higher for example), and the example of water soluble (polar for your point of reference) and fat soluble vitamins (non-polar, again for your point of reference).
A more technical answer is that you'd have to know the molecular structure of the specific plant/herb/bark/flower from which you are wanting to extract before determining the best method. There are actually more than two types of solvents- polar, nonpolar and semipolar.
There are many different variables as to how and what solvents can be used to extract certain solutes. The laws of thermodynamics for example- internal energy, entropy, and pressure, as well as temperature, can all affect solubility. Intentionally altering the polarity of the solute is one way pharmaceutical companies accomplish extraction- altering the pH of the solution for example. Another way to accomplish extraction is to mix solvents of different polarities to form an ideal sovent for the specific solute. Unless you really want a more scientific answer, it's easier to familiarize yourself w/what you need to know and have references handy. I can give you the boring chemistry lesson (unless you want me to balance equations that is ).