Sections "Daily Update" and "Trending Stories" are available for all novels and not just premium. For the Trending, some requirements are needed (number of words written and daily update). After meeting those requirements, you need to be connected to the Discord Author Server and PM one mod to be displayed. That's for the free parts.
In addition to those, you have the originals that are displayed among the translations, like "New Ongoing Release" or "Recommended". For those, only premium or "contracted" novels are able to be presented that way. So, the higher you are on the main page of webnovel, the higher the chances are that the novel is premium.