How do node_modules packages read config files in the project root?

I am creating an npm package that needs to be able to read config files from the project root. I'm not sure how to do this.

For example,

• Next.js is able to read ./pages/ and ./next.config.js from the project root
• Jest is able to read ./jest.config.js from the project root
• ESLint is able to read ./.eslintrc.json from the project root
• Prettier is able to read ./.prettierrc.js from the project root
• Typescript is able to read ./tsconfig.json from the project root
• Babel is able to read ./.babelrc from the project root

I've tried looking at their source code to see how they do it but the projects are so large that I can't find the relevant section.

How do they achieve this?

from How do node_modules packages read config files in the project root?