But Rmd already supported python chunks (as well as other languages such as ruby, which seem to be missing from Qmd)

Quarto supports any language and works just fine. I have quarto blog posts for using APL as an example of a somewhat niche language.

I guess you have to add some plugin? I mean in Rmarkdown in Rstudio I just go ```{ruby} and I have a ruby block with nothing special installed. That doesn't work by default with Quarto.

In the Quarto front matter, you can choose to use a Jupyter backend, in which case any Jupyter kernel can be used to interpret code blocks. Many languages, including APL, have Jupyter kernels you can install.