#1. What technology of transistor is best for the circuit? FET, BJT, etc.
#2. If you are using BJT's What polarities do you use more often? This determines whether you should use NPN or PNP more often.
#3. Look at how the transistor is made. a BJT transistor contains two diodes back-to back. This greatly determines what kind of transistor you use.
What is the "certain" circuit you are referring to?
Very appropriate punctuation.
Some of that was true, some was mumbo-jumbo. Unfortunately, he sometimes has a tendency to do a core dump when he sees something that he's had some experience with.
Your question doesn't have a simple answer. It takes a lot of education and/or experience to design a working circuit. If you're new, you will learn faster by copying proven designs and attempting to understand how they work.
Some of that was true, some was mumbo-jumbo. Unfortunately, he sometimes has a tendency to do a core dump when he sees something that he's had some experience with.