I'd say there's no "what if". We can already know that they both are and are not building blocks of something greater.

What are atoms building blocks of? Nothing. Combining atoms to form a parsley doesn't actually make anything new. Nothing emerges from combining atoms. There is no "parsley", there's just atoms.

Similarly solar systems form galaxies, they form galaxy clusters etc. But a galaxy is nothing but a collection of solar systems etc.

However, the direction of causation is not only from small to big. The big also influences the small, and actually also determines it. For example the position and movement of the small is influenced by the big. Its position and movement is what the small actually is, as being in a different place or moving differently it would be something else that what it is now. You can't separate a thing's position from what that thing is. And more fundamentally, a thing is its interactions with other things. Outside of its effects on other things, you can't observe a thing, and those effects are the only existence of that thing.

In that sense you could say that atoms and solar systems are not building blocks of bigger things, as there are no fundamental building blocks and no things that are built either. There is just a network of things, none of which exist as independent things other than mutually determining and being determined by the network.