The Afgans wear IRGC uniforms all part of the Iranian deniability plan; the Afgans make up only a small part of the Syrian 4th and 5th armored
The Iranians have raised separate mercenary units for service in Syria, mostly from among poor Afghan and Pakistani Shi’ites.
When Iran committed forces to shore up Assad, they took a lot of casualties, and it became a domestic political difficulty for the regime (still is a big sore point with their population). So they have rotated their regular Army units back home, and raised a sort of Foreign Legion.
The Iranian Revolutionary Guards Corps has the mission for exporting the Khomeinist Revolution outside their borders, and does this mainly by organizing local militias to operate as their surrogates. Hizbullah in Lebanon was their first, and is their best developed. They also have big militias in Iraq, that got heavy weapons during the fight against ISIS.
In the last few years, they raised a new type of units - lots of foreign troops (like Afghan Hazara Shi’ites from the refugee community in Iran), with a small cadre of Iranian officers and technical specialists.
Some of those troops may have been used to fill out Syrian Arab Army units, but mostly they are in their units, operating under direct Iranian command.
In the last month, airstrikes from Israel, and disruption in pay from financial sanctions, have been reported to have caused widespread desertions among the foreign mercenaries.