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#1. Generate a config file from within your environment:
jupyter lab --generate-config
#2. Then open the generated file
open pathtofile
#3. look for #c.ServerApp.browser = '' and edit it to:
c.ServerApp.browser = 'open -a /Applications/Brave\ Browser.app %s'
# Note: for Jupyter Notebook is: #c.NotebookApp.browser