They can have very good or very bad things in. You can safely move them to the town spire whenever you want. The key point is, once they have been understood, whether you choose to open them or not.

You will learn which results from study indicate good contents, and which indicate bad contents. And sometimes you will not know. You can play safely, or adventurously.

So they are totally random and their contents don't rely on how advanced my civilization is when I open them or finish studying them?

Thanks

Correct. For instance, a box you get at the beginning of the game may have concrete in. You cannot do anything with concrete early in the game. You cannot even store it. So it will have disappeared long before you can use it.

There's a mechanism you can use to make more boxes appear. I won't spoil what it is, but you'll probably notice it as you play the game. So you don't need to worry about running out of them.

It's a little obscure, so I'll mention it and just spoiler tag it.

Using the Meteor god power leaves a single crate at point of impact, so you can spam it to get them infinitely.