>Growing from a bad start and where you can copy a more developed nation makes this easier.

True. But you know what really makes economic growth easier? Foreign aid. Also trade.

The USSR otoh had to deal with sanctions.

https://www.americanforeignrelations.com/E-N/Embargoes-and-S...

Yet they weren't alone in the world. They were next to China and other friendlies.

China was friendly with the USSR only briefly, like in the 1950s. It was desperately poor at the time, and received a lot of aid from the USSR.

I don't know what other friendlies you mean. USSR installed communist/socialist governments in the countries it liberated from German occupation after WW2, but those relationships were... complicated.