Github project management allows us to collect issues from each repo into a central kanban management board here: https://github.com/orgs/AddRobots/projects/1
Enter issues into the appropriate repo and then place them onto the board as appropriate.
Github project management cannot have it's own milestones, and it cannot track milestones from each repo. Instead, create milestones within each repo that apply to that repo only. This is not a problem since we run sprints separately on each repo.
Jen DeWalt has an excellent guide for how to manage sprints from within Github. https://zube.io/blog/agile-project-management-workflow-for-github-issues/. We mostly follow this process, except that there are many repos across the project, so each repo has its own sprint.