Springs, bolts, screws, pins? There’s a lot of metal that’s not aluminum.
Many of these fasteners are likely stainless steel or at least zinc plated both of which would be more resistant to rust. Other binding parts are sometimes made with magnesium which doesn't rust but does corrode, so it's likely plated or alloyed with something to reduce corrosion.
Are edges stainless steel? Or some other steel?
A little research shows edges are carbon steel. Some sites stated this was due to stainless being too hard to flex with the skis, and others because carbon steel is easier to sharpen. Cost may also play into it, but the steel is not the most expensive part of the ski.
Well if the edges rust then they are not stainless steel lol
They actually still could rust if they were stainless steel. Stainless steels form a chrome oxide layer on exposed surfaces, typically during a passivation or heat treating process. Even steels that aren't stainless (>10% chrome) may form some level of chrome oxide layer. This chrome oxide layer prevents moisture from oxidizing the iron in the steel.
Damage or dings to the ski edge from rocks or similar could remove or penetrate the passive chrome layer allowing the iron in the steel to rust more easily.
This leads to probably the only practical thing I'll say here: removing large dings or burrs from your edges will also help prevent rust in your edges regardless of the steel type or composition. By removing these inconsistencies you're giving the water less places to hang out and continue to rust the edges.