Transistors are at the heart of just about any piece of electronics around us from computers to audio equipment. Here is a great video that shows the *concept* of what it is. Many people try to learn the physics and electronics of it and get lost in what it really is. Kind of like the forest from trees. A very good watch with nice history of its invention:
It is HTML 5 compatible and pretty understandable at 1.5X speed.
It is a MUST WATCH! If you know this and Ohm's law, you are way ahead of the game.
Once you have seen the above, might as well watch this on how transistors are used from the same author. He starts talking about another important concept called a "voltage divider" which has application such as volume control. It also talks about clipping that happens in amplifiers.
BTW, even "tube" devices sometimes sneak in a transistor or two in there. Here is the LAMM ML3 tube amplifier:
The two black heatsinks have "TO3" style transistors (flat stainless steal type metal) in them. There is also another tall narrow heatsink with a transistor below to the left of them. In addition, there is an integrated circuit (with parallel set of pins) above left of the power transistors. Of course many tube amps are just tubes. But I would think a designer today would take advantage of both technologies as needed.