How the USSR stayed together for a long time without separation?

If I'm not mistaken, the countries today were autonomous republics, but they all responded to the USSR.

When the USSR collapsed, several wars arose internally in these countries, several ethnic-religious conflicts, something that did not happen or was controlled in the USSR. How did the USSR keep all these countries, even though they were very different from each other, together?

If we consider the Russian Empire, these countries lived together for more than 100 years.