LaTeX is a high-quality typesetting system; it includes features designed for the production of technical and scientific documentation. LaTeX is the de facto standard for the communication and publication of scientific documents. LaTeX is available as free software.
You don't have to pay for using LaTeX, i.e., there are no licence fees, etc. But you are, of course, invited to support the maintenance and development efforts through a donation to the TeX Users Group (choose LaTeX Project contribution) if you are satisfied with LaTeX.
You can also sponsor the work of LaTeX team members through the GitHub sponsor program at the moment for Frank, David and Joseph. Your contribution will be matched by GitHub in the first year and goes 100% to the developers.
The volunteer efforts that provide you with LaTeX need financial support, so thanks for any contribution you are willing to make.