I need to make some customisation in merchant portal, and add a new controller as well as menu-item. If anyone have any knowledge or documentation reference, it will be appreciated.

Heyhey, this is what I quickly see in our docs: https://docs.spryker.com/docs/marketplace/dev/architecture-overview/marketplace-in-back-office.html

Thanks @plain-city-95899.

I have not changed the navigation in merchant portal yet - for Controllers and routes the team once told me that it works the same as Back Office but with a different naming convention (for the module itself) and then the controller etc will be auto fetched similar to back office

I tried the same approach, but the url and routes for controllers getting fetched by Spryker in its auto-generated file for routes. But, it didn't work for me. I will try it once again with documentation reference and code for mentioned modules provided in documentation.

have you emptied the cache with `docker/sdk console cache:empty-all ?

Yes, I did.