Using the explode feature will gather together and incorporate in your search all of the constituent or related parts of a concept. If you want to incorporate all constituent parts of the eye, exploding eye will give you articles on sclera, retina, etc. It's a very useful feature. Yes, but you need to know the name or number of the section. This is a useful technique if you are searching a very general term and wish to know how it is treated by specialists in any discipline.
Indexers assign the most specific term available without using broader terms.
Yes, although this depends on which vendor's version of the databases you are using. However, the general process is the same. Important similarities are that you can explode subject headings in both systems. If your results are not on topic, you may have to go back and start over by analysing your question, your strategy, your synonyms, etc. The most common error in searching any database is not considering your search topic carefully enough and not dividing it up into concepts and then not combining the concepts properly.