You need to run poetry install to set up your dev environment. It will install all package and development requirements, and once that is done it will do a dev-install of your source code.
You only need to run it once, code changes will propagate directly and do not require running the install again.
If you have set up the virtual env that you want already, take care that it is activated when you run the install command. If you don’t, poetry will try to create a new virtual env and use that, which is probably not what you want.