For example, the Romans were from a different migration (Indo european) than their very close neighbors the Etruscans. When and how did this happen, timeline wise.
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Why do you think that is the explanation of different cultures? Have you any articles that claim that Romans and Etruscans are the results of different waves of migration?
Cavemen are also not something that relay has existed. Some humans used caves for protection but others did not, they were of the same biological group. There is a simple reason that there never was a stage where humans just lived in caves, there are not very many of them.
Lost archeological finds have been made in caves, That is because there are few of them so the one that exists will have been something that could be used for thousands of years. Out in the open tents, huts can practically be placed anywhere. So it is simple to find stuff in caves than out in the open simply because of the number of possible locations.
The environment is also one that is good at preserving objects.
IE migrations, based on currently accepted, but by no means only, theory happened sometimes between 4000 and 1000 BCE, in various waves; from the hypothesized homeland north of Black/Caspian seas. The Pre-Indo European languages are not all necessarily related, but timeline wise, they predate the IE migrations in many cases. How did it happen? Gradually. People moved, settled, mixed with people who were already there, exchanged languages, goods, developed cultures, moved again etc.
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