I’m considering switching from Visual Studio Code and wondering if JupyterLab has similar search-and-replace functionality to search all files in a folder. So far I’ve seen you can search within a particular text file, but I don’t see a per-folder method.
This is tracked in https://github.com/jupyterlab/jupyterlab/issues/1145 and approved as a feature that may be merged for JupyterLab 4.0 - assuming anyone will prepare a pull request 
As for the replace part, I am hoping that the LSP extension will be able to do that too.
For now, I am mostly using grep to search my notebooks quickly, basically a saved bash script with some sane defaults for my use case of Python and R:
grep -r --include "*.py" --include "*.R" --include "*.ipynb" --exclude "*checkpoint*" $1 . ${@:2}
but I would love to have that work from the GUI.
But in case of finding a file by its name this might be helpful too: https://github.com/parente/jupyterlab-quickopen