When I was a little kid I remember that my parents gave me my grandparents’ old discarded telephone to play with. It was one of those phones that had an old fashioned electro-mechanical ringer inside it.
I took it apart and examined the innards. What I saw was very educational.
There was a metal springy clapper that was designed to hit a bell. When it hit the bell, that would connect an electric circuit that would activate an electromagnet. The electromagnet would then attract the clapper which would pull back toward the electromagnet. When that happened, the electric circuit became disconnected, which turned off the magnetic attraction.
The clapper would then fly back toward the bell striking it again. This would repeat over and over again as long as the ringing was supposed to continue.
Even today, when you hear a classic ringtone, you were hearing a re-creation of this electro-mechanical mechanism.
What was wonderful about this was that even I, a small child, could see this and trace through and understand the principle. Sadly, that is no longer the case with today’s all-electronic mechanisms.
It was still possible to reverse engineer gadgets up to 1980s. Even though most electronic functions were hidden in chips, you could look the part numbers up in catalogs and figure out what was going on. By the 1990s, customized chips hidden under epoxy blobs made that very difficult.
Of course there are still serious reverse engineers who delight in taking things a part, even if it means soaking chips in acid to get to the silicon. Ken Shirriff is a famous example (https://righto.com)