I've added a few custom fields using filter:user.custom_fields hook.
How can I expose custom fields to be used in templates?
Any response is highly appreciated.
Which template file exactly? You'll probably have to look into one of the hooks to add data to that page:
Node.js based forum software built for the modern web - Hooks · NodeBB/NodeBB Wiki
For example, I would like to replace username on topic, like shown on the screenshot: