This repository contains the worksflows that are used to sync, build, check, and deploy R packages on R-universe. This has been set up using so-called reusable workflows:
The actual runs and jobs are triggered in the monorepos of the universe owner under the r-universe GitHub organization. For example runs for ropensci can be found at: https://github.com/r-universe/ropensci/actions
Each of these monorepos contains a copy of the build.yml and sync.yml templates from this repository. However these are mereley small wrappers which invoke the actual "reusable workflows" that you can find under .github/workflows in this repository.
If you are here to find out how R-universe builds packages, or want to suggest changes, you are probably looking for the .github/workflows/build.yml workflow. This file contains the full process to build, check, and deploy each R package update.
The entire process is open-source by design, and you can easily find the source for each "action" used in the workflows by navigating to the respective GitHub repository. You will find that the build workflow uses a lot of actions from the repository r-universe-org/actions. This is where you could suggest changes to the individual steps of the build process.