In Gephi, network open, file -> export -> sigma.js template, specify a folder... and nothing is there when I look. I thought maybe it was somehow invisible and went ahead and used python -m http.server in the folder it supposedly exported to and went to localhost:8000 (according to the instructions of this tutorial: https://blog.miz.space/tutorial/2020/01/05/gephi-tutorial-sigma-js-plugin-publishing-interactive-graph-online/ and others), but there was nothing there. I also have sigma and its dependencies installed in that particular folder for what it's worth, although I don't know if that would be required for the file to appear. Btw I am very unfamiliar with js so if this is some js wizardry, explain it like I'm a baby.
There isn't a single file that is produced. It is a folder called network containing different files/subfolders including index.html, the sigma module, an images folder, a css folder, a js folder, a README, web.config, data.json, config.json. To use it in the browser navigate into the network folder and launch a server vs php -S http.server.