I usually use my browser to go to the server which shows the file tree with notebooks (like http://remote-host:8888/tree), and I select a notebook from there and work with it.
In this case, the URL to a notebook is like http://remote-host:8888/notebooks/xxx.ipynb.
Note that the notebook itself is truly remote, i.e. any changes are saved on the server, not locally inside PyCharm.
How can I connect to that remote notebook from PyCharm?
How can I get a list of available notebooks?
Looks like most of the posts here are about creating a new notebook inside PyCharm and run it on a remote server, not about actually using an already existing remote notebook, unless I'm missing something.