Century eggs may look odd when you first encounter them, but they've been a beloved ingredient for hundreds of years.
Traditionally, century eggs are chicken or duck eggs preserved in a mixture of clay, salt, lime, and rice hulls. Over time, mass production has modified the preservation method with different or simplified ingredients, and cut down the curing time to months, or even weeks. The result of preserving eggs in this unique, alkaline mixture is a jelly-like black-brown egg white, and a dark green egg yolk.
Century eggs are strongly savory, and their texture is just like boiled eggs, with creamy yolks and solid whites.
Using dried scallops
Dried scallops have more concentrated flavor than fresh ones, so while you could use fresh seafood in a congee, it wouldn't have that intense fragrance that dried scallops provide.
Just like other dried ingredients, you usually want to soak them in water to soften them up before you start cooking. For this recipe, since they'll be hanging out in a big pot of liquid for most of the cooking time, there's no need to pre-soak!
Pork butt or pork shoulder?
When you buy meat, you might see both names and think that pork butt comes from the buttocks, and pork shoulder from the shoulder. Unfortunately, the naming convention of pork cuts isn't straightforward and sensible like that.
Pork butt is actually a marbled cut from the pig's shoulder. If you get a ham cut from the pig's buttocks, or "pork shoulder" which is a leaner section of the pig's shoulder, it'll taste fine, but the meat will not be as tender as if you used a marbled piece of pork butt.
You can use other cuts of pork, or other meats, but you'll want to get something that's balanced between lean and fat. Too lean, and it won't be tender and juicy. Too fatty, and your congee will get greasy and oily.