They’re still there… just hidden.

Traditionally, stereo speakers didn’t contain any electronics, you had to use wires to connect them to the “Speakers Out” terminals on your device. Those Speakers Out terminals are the output of the amplifier. 

It meant you needed wires, but it also gave you freedom to position your speakers for best sound, best fit with furniture, etc.

More recently, all the electronics are frequently built in to the speaker, rather than being  separate boxes.

Think of a bluetooth speaker, or a soundbar. You’re playing digital media through it; maybe a streaming movie soundtrack, or Spotify, or similar. Same as a vinyl record, this is a tiny electronic signal that needs to be made bigger, to play through the speaker.

Inside the speaker box is the signal source (maybe a bluetooth receiver), and small amplifier, that does the job of adding enough power to drive the speaker.

And if you listen to bluetooth headphones, there’s a tiny amplifier built right into them too.

Even in your phone, if you listen to headphones plugged right into your phone… the phone itself has a little amplifier in it just big enough to drive the headphones.

So how can you spot where the amplifier is hiding in modern gear then? The dead giveaway is the presence of a Volume control. If you see one of these, it’s the part of the amplifier that you can see and interact with.