So that if, for example, i put two cubes of copper next to each other they don’t become one
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Everyone is explaining cold welding because you mentioned copper. The answer to your question more generally is something called “van der Waals forces.” The ELI5 answer is that if molecules get too close, their electrons start repelling each other extremely strongly. There are other factors that go into it, but basically that is what we care about here. So why is your copper example different?
For any non-metallic object, the electrons are bound rather tightly to the nuclei or in the molecular bonds holding that molecule together. Since specific electrons more or less “belong” to atoms or bonds, they don’t have anywhere to shift and will repel other electrons. This is why generally similar materials don’t just randomly fuse together.
For metals, the bonding between atoms works a little differently and electrons are shared more or less freely between the individual atoms in the metal. In a perfect world, two copper cubes would bump into each other and the electrons would repel each other at first but they have enough wiggle room that the other cube would just kind of join together. This is essentially cold welding, the “perfect world” being a vacuum with very clean copper.
In the real world, basically all metallic objects have a thin oxide layer formed around it from the raw metal reacting with the air around it. Oxides are non-metals and have ionic bonds, rather than metallic bonds. This effectively holds surface electrons in place and the surface of the two cubes effectively repel each other. Add on the fact there’s probably dust or grease on the surface as well, and you can see why they don’t just spontaneously fuse together.
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