The Wolfram Engine (essentially the Wolfram Language interpreter/execution environment) is free: https://www.wolfram.com/engine/. You can download it and run Wolfram code.

Wolfram Mathematica (the Jupyter Notebook-like development environment) is paid, but there are free and open source alternatives like https://github.com/WLJSTeam/wolfram-js-frontend.

> WLJS Notebook ... [is] A lightweight, cross-platform alternative to Mathematica, built using open-source tools and the free Wolfram Engine.