is the project ID which you need to pass into the API request URL. That can be seen in the project overview in the UI, or by searching/getting the project via API.
For the API request itself, the endpoint is constructed with /api/v4/projects/13083/repository/files/ and the file structure underneath.
If I navigate to a specific file, https://gitlab.com/everyonecancontribute/web/everyonecancontribute.gitlab.io/-/blob/main/config.toml I know that /blob/main is the reference to the current branch main and everything after it represents the relative file path.
It is manual work though, and I try to avoid constructing URLs without first having searched and matched a pattern, e.g. fetching all files with the *.toml or *.md file suffix.