Grand Piano Action
The picture on this page shows an example of a single key in a grand piano action.
There will be subtle differences between actions built by different manufacturers, but the basic premise is well represented here. The Renner action company is one of the biggest, there are others. The U.S. used to have an action manufacturer in Connecticut called Pratt-Read (they also made keys and key tops, in a town called Ivorytown, CT) but they have gone out of business.
Grand actions tend to be faster and more responsive than vertical actions, primarily because of the “repetition lever” (#19 in this picture) which resets the jack (#31) when the key is only about half way up, positioning it where it can lift the hammer (#6) again. This allows the pianist to repeat a note when the key is only about half way up, unlike the vertical (spinet, console, studio, upright) piano action which requires lifting the key all the way up in order to reset the mechanism (jack) necessary to repeat the note.
As you can see there are a lot of moving parts in the grand piano action, and a number of points where the action can be adjusted (regulated). In many cases one adjustment can affect another one requiring a number of small incremental adjustments until everything is working properly together so that the pianist has the control and “feel” they desire/expect from the piano. Multiply this procedure times 88 (the standard size piano keyboard) and you get some appreciation for why it takes time and money to do this properly.