I am starting with nextcloud version 28. I wanted to know is it possible to do small customization in files app like making a new folder as default & hiding some navigation tabs.
Yes, Nextcloud is open source and you can change the code.
If you speak of the default files/folder for a new user:
https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/default_files_configuration.html
There might be even an app customCSS app:
and people were asking e.g. to remove the download button:
So depending on the element you want to hide, you need to be able to address this via css.