You’ve got a number of options here but it’s very much down to how your repo is structured, you’re absolutely right - there’s no right and wrong way.
Whilst we don’t have a specific guide, some googling led me to:
https://dev.to/gregpetropoulos/render-deployment-free-tier-of-mern-app-52mk
and something else you might want to consider is using our Monorepo https://render.com/docs/monorepo-support feature which lets you specify which folders are deployed to which service but that depends on your folder structure.
John B