Terrace and driveway

As I have already said 3-4 times, no current professional substructure allows joint vegetation from below. The joint vegetation comes from above and is absolutely unavoidable. If in one place you have no plants on the joints, there are other reasons for that (no light, constantly driving over it with a car, etc.).
Where do the ants come from? If they are walking on top and running into the joints, that also has nothing to do with the substructure.

And I write that the only difference is the substructure....

One time there is vegetation and ants (sand) and the other time not (gravel)...

And I write that the only difference is the substructure....

One time there is vegetation and ants (sand) and one time not (gravel)...

I would also be more cautious here - a coincidence does not always have to mean causality, more factors may also play a role!

And I write that the only difference is the substructure....

Once there is vegetation and ants (sand) and once not (gravel)...

you are basically saying something like:
I have a red and a blue car. I crashed the red car, so every red car is shit.

as saruss already wrote, there can be other reasons. nothing definitely grows from below. the grass growing on the stones of the lawn clearly indicates that seeds have gotten into the joints. ants can also walk on paved roads, and the joints themselves are also damaged from above. they definitely do not come from below.

No, I certainly did not write that. If you read it that way, I can only recommend you read everything again.

So - a few cobblestones are already laid and now I'm a bit unsure. The row directly next to the house is laid with granite - it otherwise borders the paths and looks quite good. But at the house we have 3 "colors"... on the other side it separates the terrace from the house. Should we replace them or keep them like this? They are not fixed yet.