It's selection by the elected officials of the parties in the majority coalition.

In a single party situation, elections are multi-candidate where there are multiple candidates available - this was seen in the USSR and is seen in China, and like the USA or many places in Europe, some races only have one candidate just simply by virtue of the office.

There are other offices that are not selected by popular vote. All democracies have these too, both liberal democracies and Marxist democracies. Some of these offices are appointed, some of these offices are elected from within specific committees or other bodies.

You can study the Soviet system, too, ya know. It's not illegal. You can learn all about the workers councils, the right to recall, the Congress of Soviets, etc.

Of course, in 1936 they got rid of the Congress of Soviets and added universal direct sufferage well ahead of any liberal democracy, and they added the right to rest and leisure, right to care in old age and sickness, right to housing, etc.

It also constitutionally enshrined the equality of women and of all peoples regardless of nationality/race. Very fascist, I know.

What was very anti-democratic was when Stalin a year later had been purging political opponents and made all the elections single-candidate elections because he was "paranoid" about the emergence of counter-revolutionary powers taking over.

I put paranoid in quotes because immediately after Stalin left office, Kruschev took over and began the liberalization of the USSR complete with attempts to collaborate with the West in an effort to divide up the world and build a Soviet Empire. So Stalin wasn't wrong that there was a large threat of counter-revolutionaries trying to get power. He just failed to stop them from taking power. He only delayed them long enough for the USSR to save Europe from the Nazis.