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This is one of those $1,000,000 questions and one which is difficult for those of us who are only somewhat into electronics, like me.

I think the basic answer is yes, they can be used to do the same job but other things will determine which is best to use. By best, I mean many things, for instance, there may be no electronic reason for using one over the other, then again there could be. It might be easier to use one over the other because of what is being done, see Below. There could also be a cost reason, one way might require more parts than the other to make say a PNP work in the same job as using an NPN, or vice versa.

I looked this up because it’s a question that has always bothered me that I don’t really know, I just think I might know.

Below:
Here’s a site that helps: How to decide between PNP and NPN https://www.controldesign.com/articles/2016/how-to-decide-between-pnp-and-npn/

While you may not understand everything said in the above article, I don’t either, it does hit upon my two reasons, one is easier to use than the other, one uses more parts than the other.

For a home circuit this may not matter but in industry where extra parts means much more overhead cost to make something, it does matter.

Also, as in software, sometimes it’s easier to understand a circuit of one type rather than the other. I had this experience in software development. I had two developers who know some really interesting ways of doing things, but no one else could understand the method as it was written. My feeling was that if that person ever left the company no one else would be able to debug such a section of code, so I kept lobbying for code that was easier to understand by a majority of software developers.

Such is also the case with electronics, while it might be possible to do it that way, the other way, - NPN vs PNP - might be much easier for a majority of electronics engineers to understand, so that’s the one you should use.

I hope that helps, it did help clarify things for me.

Mostly, yes. There are not that many general types of transistors. NPN and PNP bipolar transistors are by far the most common. They are used for small signal amplification and switching.

If high frequencies are involved, you must use a type with useful gain at the frequency of interest. If high power is involved, then that must be considered. Some circuits use higher than common voltages, and high current, and that must be observed.

Darlington types are used where extremely high gain is needed, but the small differences between the thousands of types are simply not that important. I have only a very few that I use for everything.

Then there are MOSFETs which are mostly used for high power control of motors and such. One big consideration here is the maximum voltage and current and how you provide heat sinking. Specialty types such as enhancement mode FETs and Unijunction are not that common, and there are not that many different types available.

Mr. Bodine raised a question about transistor choice and interchangeability for a given circuit. In short, choice depends upon purpose, circuit polarity, operating characteristics such as signal input voltage or current, power-handling, operating temperature, power supply, etc. etc. Some circuit designs are more amenable to using MosFETs rather than bipolar junction transistors, and vice-versa.

The book that launched me into transistor circuit design is by Albert Malvino, entitled Transistor Circuit Approximations. While it is currently out of print, a number of used copies are available through Amazon beginning at under twelve dollars.  See https://www.amazon.com/Transistor-circuit-approximations-Albert-Malvino/dp/007039878X.

There are generally two types of bi-polar transistors. PNP and NPN. Most small signal (i.e. not hi power) transistors can sometimes be substituted for a like transistor. That is a PNP can be replaced by a PNP but NOT a NPN. Likewise a NPN can be replaced by another NPN but NOT a PNP. There are lots of cross references to look up equivalent replacements. My favorate is https://www.nteinc.com/ I may be mistaken, but I think the most popular type of transistor in use today are of the NPN type.

The type of transistor used depends on what your doing.(amp, buffer, volt. regulator, ECT.) Also circuit specs. PNP and NPN can be swapped if the emitter and collector connections are reversed. This is true so long as device/circuit specs are similar. Hope this helps.