Question: why did the countries and continents seperate in the first place

This a pretty interesting question. The prevailing view is that plate tectonics causes the continents to drift relative to each other, apart from each other in some cases and toward each other in other cases. This explains everything we see in terms of the continents. The continents are big bits of the Earth’s crust, floating on a very hot, molten material that lies underneath the crust. It is a bit like watching oil on the surface of water as the water heats up. Globs of oil move around, separate and combine. Similar physics going on. There are some very radical views out there regarding the continental separations. I’ve seen pretty convincing analyses that show that the outlines of the continents fit together better if they completely covered the Earth’s surface at an earlier point, with the Earth significantly smaller than what it is now!! This is quite bizarre, but I’m not sure it is so easy to prove that the Earth has always been the same size. Of course this raises piles of questions about how the Earth might increase in size over time. But science is about assessing seemingly radical views and seemingly sensible views side-by-side. All views are valid until they can be shown to be incorrect, either by experiment or from first principles.