Ooh this was my most recent special interest! My source is the podcast Revolutions by Mike Duncan which I highly recommend if you like hyper detailed history. But here’s my summary:

1. at the end of the 1800s there are a bunch of leftist groups fighting the Tsarist government. They broadly fall into 3 groups: the SRs (short for socialist revolutionaries), who want an agricultural, decentralized form of socialism; the Marxists, who want an industrial, authoritarian form of socialism; and the liberals, who want a western style democracy or constitutional monarchy.

2. the Marxists split into 2 political parties: the bolsheviks, run by Vladimir Lenin, who want a very orthodox form of Marxism, and the Mensheviks, led by Julius Martov, who want a more lenient, broad umbrella of Marxism. Trotsky begins as a Menshevik, then becomes a Bolshevik.

3. in 1905, a half-revolution forces the Tsar to accept a parliament, called the Duma. This Duma becomes dominated by liberals, who form a party called the Cadets. In 1906, however, the Tsar makes some laws that basically make the Duma mostly powerless and advisory. In this Duma, Alexander Karensky becomes the leading figure of liberals and moderate leftists. There is also a moderate party called the Octoberists who think the 1905 revolution was the only necessary revolution, and there are absolutist conservatives as well who don’t really have a named party.

4. in February 1917, a mostly leaderless mass movement overthrows the Tsar. Because it’s leaderless, there is no plan for what comes next. The liberals go to the Duma and say “ok now the Tsar is gone you’re in charge”. The leaders of the Duma declare themselves to be the Provisional Government which will rule til they can organize elections to a committee to create a constitution. After some shuffling and chaos, Karensky comes to lead the provisional government.

5. meanwhile, leftist go to working class neighborhoods, factories, and military barracks and say “now that the Tsar is gone we’re all in charge. Elect representatives to go to a council of factory workers and soldiers which will be the new government”. That council is called the Soviet, and neither the Soviet nor the Provisional Government want civil war so they agree to a vague undefined power sharing agreement til the constitution can be written and a final government made. at the top of the Soviet, which is a gigantic body of hundreds of representatives, is the Executive Committee of the Soviet, which is small enough to actually make decisions.

6. SRs, Mensheviks, and cadets join the provisional government. SRs and Mensheviks also join the Soviet. The Bolsheviks only join the Soviet and say the provisional government is illegitimate.

And that’s February 1917. There will be more complications as you go but that’s an oversimplified summary. Hope this helps, happy to answer follow up questions!