Almost every switch you'll find in a custom keyboard comes in one of three categories: it's made by Cherry, based on Cherry designs, or at least compatible with Cherry parts. None of this is true of the switches manufactured by Japan's Alps Electric. This company made keyboard switches for decades but discontinued them several years ago. Still, keyboard enthusiasts scrounge up old, damaged boards just to harvest Alps switches. Why? They're pretty interesting.
Alps come in various types, but it's the SKCL or "complicated" Alps that get the most attention. Alps Electric introduced these switches in the early 1980s and discontinued them in the mid to late-90s. As you can tell from the above image, Alps are completely unlike Cherry switches. Back in the day, Alps were an alternative to Cherry that required different keycap and PCB designs. Cherry won that battle, so Alps are rare today.
Complicated Alps are so-named because they're, well, complicated inside. Depending on the switch, there are around ten parts in an SKCL Alps. Cherry-style switches have five if you count the top and bottom housings as separate. Alps came in all sorts, just like the simpler Cherry switches. There are clicky, linear, and tactile Alps with various spring weights. Like Cherry switches, Alps are identified by stem color. The switch I've used here is an orange Alps, which is a heavy tactile switch.
In Cherry switches, the switch's properties (e.g. clicky or tactile) come from the stem. In Alps, it's all about the contact plate. Change the plate, and you can make a linear switch into a clicky one, Tactile into linear, and so on. The stems are mostly identical aside from the color.
Alps switches showed up in a lot of early Apple mechanical keyboards, so these devices are highly sought after. Alps switches aren't the smoothest, but the tactile switches are very tactile. The clicky ones also have a sharp, loud click that Alps fans prefer to Cherry-style switches. There are also "damped" Alps switches that have rubber bumpers on the stem to make them quieter, similar to the Zilents I covered recently.
One significant drawback of the Alps design is that it's easier for dust to get into the housing around the stem. Salvaged Alps often feel gritty because of this, so you need to take them apart and clean the various pieces. It's time-consuming, but people do it because they love the switches.
If you want to experience classic Alps switches, you'll have to find some salvaged keyboards online. Then, you need a custom board that supports Alps. Most fancy boards don't work with Alps, but there are a handful like the XD75 and S65-X.