Russians With Attitude@RWApodcast
It's a very long story.
For example, in Aleppo, the Iranians manufactured disorder and staged violence against civilians specifically to disrupt Russian military police operations, with Iranian proxy forces sometimes even attacking pro-Russian Syrian paramilitary forces. They held Aleppo airport outside of Russian coordination and used it as a corridor to Iraq to bypass Russian oversight. They increased pressure until Russian military police withdrew from Aleppo, and the disorder and clashes stopped overnight. It was a sustained Iranian anti-Russia operation. This happened in other places, too. The goal was to have Iranian proxies have control instead of Syrian government forces.
They kept the Fourth Division of the SAA entirely out of Russian restructuring efforts and turned it into a parallel army and an Iranian proxy force. The Iranians ran the Syrian National Defence Forces as an Iranian puppet army and steered the government to feeding recruits into the NDF and various Iranian proxy militias instead of the SAA and particularly the Fifth Corps, which was a Russian attempt to build a real, professional Syrian fighting force. Syrian Generals on Iran's payroll actually STOLE entire weapon shipments from the Fifth Corps and gave them to Hezbollah or sold them to the black market.
They used political intrigue to push aside Syrian figures such as Asef al-Dikr, who was the key Russia liaison to reforming the Syrian Arab Army. They used their influence in Damascus to appoint Iranian puppets to important posts to block Russian institutional penetration, also preventing any systemic reform of the SAA.
That's just a small part of the military aspects; there was also the 2018 deal the Iranians forced upon Assad that gave Iran exclusive rights to rebuild the Syrian military-industrial complex, fucking with Russian efforts even more. You could write entire books on the various ways the Iranians and pro-Iranian elements in the Syrian government prevented Russia from fixing anything long-term.