My knowledge of communism is probably lacking, but my basic understanding is that in a communist system, the people produce goods and hand them over to the state, and the state distributes them equally among the people.
But the USSR had a currency, the Ruble. I would expect the existence of a currency would imply capitalist forces at work.
Where did people get rubles?
Would they have to spend them in state-owned stores?
Was there any "grassroots capitalism", like people using the rubles to buy and sell from each other directly?
The ruble was simply a medium of exchange. It does not really imply that capitalist forces being at work.
Where did people get rubles?
They were paid wages, usually. The employer (a state-enterprise enterprise) would be in charge of distributing cash.
Would they have to spend them in state-owned stores?
Most goods would be bought in state-owned stores. Though people didn't really "have to" spend them - they could put them under a mattress, in a savings account or even bought government bonds (though the interest on those things was basically zero). One could also spend the rubles on the black market (which was not state-owned, obviously).
Was there any "grassroots capitalism", like people using the rubles to buy and sell from each other directly?
Yes. People bought and sold directly to each other. People couldn't own certain things (like businesses or real estate) and, of course, one couldn't buy/sell things that belonged to the state. But there were markets for, for example, produce that people grew in their gardens, used clothes, household items, etc.