Servo motors, solenoids/actuators and magnets and simple bracket design. It got pared down over time to simplicity of function and hence lower cost BOM and reliability. There have been some cool features in later years leveraging that same technology, movie screen ramps in Stranger Things, super charged loop in Star Wars, the ship in JPPOTC, ball thrown in Aerosmith and Houdini etc. That tech works and is reliable and allows for a range of features like that.
The competition between pin manufacturers is what will drive features. Competition also made the LCD get adopted by all of the pin manufacturers so that happens too. Stern developed the Spike system under-the-hood for that. Not gameplay innovation in that regard, but the LCD innovation is one important change in modern pins.
Software has changed a lot since 1999, more memory more CPU. That's huge for gameplay rule innovation, music and callouts too I guess.
MM castle bridge moves via a servo motor and the castle gate moves by a solenoid and actuator. The castle towers move via a solenoid and actuator and levers. It's simple and elegant. I own that B/W castle mech production drawing. Cool stuff.