I’m new to customizing Rocket Chat for our office; how can we do it, and can we sell it as a product afterward?
You’ll probably need to fork it.
Please read all the devs including the dev docs.
Also remember that open source is a two way street. If you fork it and profit then contribute something back to the community. Don’t just take.
I cloned RocketChat and ran yarn install, but I’m seeing many deprecated warnings and errors. How can I properly run and customize RocketChat with the full source code instead of the bundled version?
Please read the documentation.
No one is going to give up their time to tell you how to build this and make profit for your own benefit.