I have wondered how scientists figured out how various chemicals interact with each other in our bodies or in some cases even within cells? Is this all done within test tubes or other similar containers. for example I take insulin and in the 1920s scientists won the Nobel prize for finding out that type 1 diabetics need insulin to live. Prior to that you did not live long without insulin. BTW all insulin in the US is made in a lab now. they no longer get it from animals.
This is a very big question, and so of course the answer is also very big. Some experiments are done in vitro (“in glass”), and some are done in vivo (“in life”).
One extremely useful technique is isotope tracing. You take some molecule used in living things, and replace some of the atoms with uncommon isotopes of those same elements. Different isotopes of an element behave (almost) identically chemically, but they can be distinguished by mass spectrometry or other modern techniques. So after you’ve put the altered molecules in the living things, you look to see what other molecules those isotopes end up in.
Not sure if you meant for ex-vivo to be taken as part of in-vitro; it’s worth mentioning separately (IMHO) - kinda the half-way house between in-vivo and in-vitro. Has much of the “naturalness” of the former, but with much of the control and enhanced reproducibility of the latter.
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I know rats and mice are used a lot. You might be surprised that bacteria are actually involved in early tests for drugs to see if they are safe even before rats and mice