First, just a bit of a disclaimer, studying Russian intelligence history is hard. Really really hard. Russian sources from the intelligence world are basically near zero, unlike in the West where we have a pretty clear understanding of how our intelligence agencies evolved over time. Robert Pringle once compared the study of Russian intelligence and its historiography to something akin to Egyptology, rather than studying modern history. With that being said, your question can be answered.

Modern Soviet intelligence services arose out the CHEKA (All-Russian Extraordinary Commission) which Lenin founded in 1917 to serve as the internal police/counter-resistance organization in the wake of the revolution. It was placed under the command of Feliks Dzerzhinsky, a Pole of noble descent turned revolutionary agitator, who had spent the vast majority of his life either in exile or in prison. Under Dzerzhinsky's leadership, the CHEKA became an invaluable tool for Lenin, essentially cleaning house and maintaining a pretty constant rate of executions for anyone who was a potential threat to the new regime. This included ferreting out former Tsarists and anyone who seemingly was ready to hamper Lenin or his allies in their plans. With the worth of his organization demonstrated, Dzerzhinsky set about expanding the CHEKA's authority and power. He grew the bureaucracy of the organization to a staggering 250k individuals by 1921 (the Tsarist Okhrana, by comparison, had only 15k members at its peak). That bureaucracy included military units, secret police, actual police, spies, prison guards and administrators, former Okhrana turned Bolsheviks, gangsters, military political officers, criminals, and other auxiliaries. This massive pool of resources made it easy for the CHEKA to identify effective and/or experienced individuals and ensure that they were placed in the positions were they could provide the most benefit. As such, the CHEKA was able to ultimately take control of all branches of intelligence, counterintelligence, and border security, in addition to its duty as the internal enforcers and executioners of the revolution. The CHEKA, through trial and error, laid the groundwork for the operations that became the hallmarks of the Soviet Union's intelligence community: brutal counterintelligence operations and cunning use of Human Intelligence (HumInt). Given its almost unlimited power , the CHEKA set the standard for turning the Soviet Union into a state that was all but impenetrable to outside intelligence agencies. Anyone suspected of being a traitor was almost immediately executed, imprisoned, or placed in a position from which they could not rebel (for example, holding their family hostage as a guarantee of good behavior). In terms of HumInt, the CHEKA used their power to terrify and coerce individuals into forming a massive network of informants within almost all areas of the Soviet Union's government and society. This network also logically extended to foreign countries, often through the use of contacts within sympathetic political parties, worker's unions, or communist activists, most of whom did not need to be forced into assisting the Soviet Union.

The CHEKA was ultimately folded into the NKVD, which by 1931 had, according to Pringle, 27k paid informers and almost 279k "volunteer" informers, a network that ultimately expanded into the millions by the time Stalin finally died. Stalin fostered the NKVD and actively engaged with its leader, ensuring the secret police and intelligence agencies were always on his side. With that patronage, the NKVD expanded and developed the methods of the CHEKA, further improving on that organization's playbook and perfecting it. When the NKVD eventually evolved into the MGB and then the KGB, the intelligence community was well-established, highly effective within its chosen domains, and ultimately on track to become the largest intelligence agency in the world by the 1970s.

In short, the answer to your question is, give an organization near unlimited power and massive resources and eventually, under ruthless leadership, they will find the people they need to fill the roles they need.

Sources

"The Intelligence Services of Russia" by Robert Pringle in the Oxford Handbook of National Security Intelligence

The Sword and the Shield : The Mitrokhin Archive and the Secret History of the KGB - Christopher Andrew and Vasili Mitrokhin

Historical Dictionary of Russian and Soviet Intelligence - Robert Pringle

KGB: The State Within a State - Yevgenia Albats

The Cheka, G.P.U. and O.G.P.U.: Bolshevism's Early Secret Police - RJ Stove