It's colloquial and you'll hear it when people talk about making food. Like if you're making a sandwich. You put mayo on the bread, then you put the cheese, then you add meat and lettuce or whatever.
It's kind of like "on" is implied and you don't bother to say it. I just mentioned it, so I don't need to say it again. That's how it feels to me anyway.
I could see myself saying "First you put mayo, then you put cheese." That would be like if someone was standing next to me, watching me make the sandwich. They can see exactly where I'm putting things. But normally you do want to specify where you're putting something.
It depends. Cheap salt is just fine. And flour, unless you're into baking. But some things can make a difference and you don't necessarily have to pay a lot more for it.
Pasta, for example. Bronze cut pasta absorbs sauce a lot better than "normal" pasta. It looks dull, rough, and pale as opposed to shiny and smooth. It usually only costs a buck or two more. I find it's a big step up taste and texture-wise.
Or butter. The ones without natural flavor taste better. Sometimes it's the store brand that doesn't have added flavor.
And eggs. Orange yolks are way better than the pale yellow ones. But those you do have to shell out for.