As explained here.

Each of the first few movies succeeded in pushing back the creation of Skynet. However, in the process of doing so, some of the technology was left behind in the past. Liquid terminators were possible because Skynet from that movie actually was more advanced than Skynet from the first movie.

Skynet would have still happened over the course of time, regardless of what was sent back.

Terminator 1's Skynet was the "natural" one. It progressed without advanced Terminator technology being the basis.

Terminator 2's Skynet was more advanced than Terminator 1. They started with the chip and robotic arm as the basis of technology.

Terminator 3's Skynet was more advanced than Terminator 2. Although no parts were salvaged (everything melted), ideas inevitably sprung into the minds of developers when they saw news footage of a liquid-metal-man. They might have even been able to separate the Liquid Terminator metal from the normal kind.

As explained here.

Each of the first few movies succeeded in pushing back the creation of Skynet. However, in the process of doing so, some of the technology was left behind in the past. Liquid terminators were possible because Skynet from that movie actually was more advanced than Skynet from the first movie.

Skynet would have still happened over the course of time, regardless of what was sent back.

Terminator 1's Skynet was the "natural" one. It progressed without advanced Terminator technology being the basis.

Terminator 2's Skynet was more advanced than Terminator 1. They started with the chip and robotic arm as the basis of technology.

Terminator 3's Skynet was more advanced than Terminator 2. Although no parts were salvaged (everything melted), ideas inevitably sprung into the minds of developers when they saw news footage of a liquid-metal-man. They might have even been able to separate the Liquid Terminator metal from the normal kind.