You may find the following resources helpful to prepare for an R Dev Day.
Reviewing Bug Reports, R Blog post (3 min read)
Debugging in R (1h45 video)
Analysing Bugs/Contributing Patches (2h15 video + exercises - assumes some knowledge of debugging in R).
Starting Codespace and Stopping and restarting, to learn how to use the R Dev Container through the browser with GitHub Codespaces, followed by Running R, Building R and R Contribution Workflow to learn the basics of using the R Dev Container to develop changes to base R. (<1hr step-by-step guide).
R Dev Guide (ebook). This is designed as a reference for contributors on different aspects of contributing to R. In particular see Chapter 2 for learning how to build R on your own machine. Currently the instructions for macOS have not been incorporated, but there is a draft version.
Contributing translations to R (1h video), tutorial introducing translation via the Weblate browser interface.
Learning to build R in the GitHub Codespace (i.e. in an instance of VS Code you can launch through the browser) or on your own machine is recommended for all participants of R Dev Days.