This is usually done via the "fuzzy finder", with Ctrl + .. Note that this navigates both function definitions as well as files located in the active project.
Lines and be commented + uncommented with Ctrl + Alt + C.
There isn't a preset keybinding for this, but you could add your own keybinding via Tools -> Modify Keyboard Shortcuts, with a binding for the Insert Chunk Python command.
It seems to work for me (macOS 11.0.1; RStudio 1.4.1069). Would you mind filing this as a bug report at Issues · rstudio/rstudio · GitHub, with some extra context on what exactly you're seeing?