Question

How did helping the Taliban defeat Russia during the Afghanistan War help the Taliban in their efforts to resist United Nations forces today?

Answer 1

Your question is flawed. I'll explain.

The US supported the Mujahideen when the Soviets invaded Afghanistan. It was one of the last theaters of battle in the Cold War.
Mujahideen is plural for jihad warrior in Arabic. In 1980s Afghanistan, the Mujahideen was not one force. It was a group of tribes that opposed the puppet government that the Soviet Union had put in power in Kabul.
(A puppet government is one that looks like a real government but someone else is pulling the strings. In this case, Moscow was really in charge.)
The Russians got tired of fighting all the different tribes who were getting help from the CIA and our allies. So they left.

The tribes were not united. That was no surprise. They'd been arguing and such for centuries. But now they had lots of guns and stuff left over from the Americans. And the Americans left to go fight wars in Bosnia, Somalia and the like. So it left a power vacuum in Kabul.

While the Russians were in Afghanistan, some refugees went and lived in Pakistan. The children of those families grew up in Pakistan and many were trained in Wahhabi madrassas (schools). Wahhabi is an extreme version of Islam supported by many Saudis and their oil money.

A group of the now young adults looked at the confusion in Afghanistan and saw an opportunity to take control using Saudi financed weapons and Toyota pickups. This group had been students in the madrassas, so they took the name Taliban for their group. Taliban, in Pashtu, means students.

So, the US did not help the Taliban in the Afghan War. The Taliban did not form until later.

The US did help the people who got the Soviets out. Then they left. It gets messy when you go into another country to help. And Americans don't usually like to stay in that country for very long after winning. Examples: Iraq, Viet Nam. But those are whole nother stories.