There are a ton of R packages in Nixpkgs already! To use them with R, you ask Nix to 'build' an R that has them available. This is the general pattern for all languages available in Nixpkgs, but the details vary between languages somewhat. You can find the guide for any/all of them in the Nixpkgs manual. Here is R's: https://nixos.org/manual/nixpkgs/stable/#r

I think not literally everything (especially if it has native extensions) is necessarily working, but everything in CRAN and Bioconductor is on there. Give your faves a try!

To explain this seemingly strange behavior a bit: because of Nix's commitment to isolating dependencies, when you install a language interpreter to some environment, it needs to behave the same way no matter what is installed in that environment— including packages from its own ecosystem. So instead, Nix has you be fully explicit ahead of time about what you want each language interpreter to pick up on, and those are the only in-ecosystem packages it'll ever be able to see.

This is super nice for per-project dependencies and for scripts that can bring along all their own dependencies, but it can seem a little weird when what you're used to and what you expect is just managing a single environment.