@mstan — sorry that you haven't received a response.
NodeBB doesn't save the Mongo database anywhere specific on the file system, it would be wherever the insulted MongoDB install saves it's data.
I need to host my forum in http://xyz.com/forum while xyz.com will direct to the main site. Is there a tutorial for this purpose? Can I just redirect all the /forum into nodebb and done with it? (I'm new to NodeBB)
Yeah essentially you just set the correct url with your /forum
path in config.json and set your reverse proxy to forward that path onto NodeBB: