How to better isolate a floor with carpet?
When you install carpet you're first supposed to put down a carpet padding. This is available in a number of different materials including urethane foam and rubber though most commonly these days I see a recycled urethane product. It's main purpose is to prolong the life of the carpet by absorbing some of the shock of being walked on. It also provides a softer feel underfoot as well as some insulation.

If you want to increase the R value of the carpet padding, choose the densest and thickest padding recommended by the manufacturer for the carpet you're buying.

The Carpet Cushion Council (apparently it's a thing) has some example R-values for carpet padding listed on its website.