When you assign unique permissions to the list, the items in the list will also automatically inherit the unique permissions of the list.
If the list inherits the permissions from site collection and does not have unique permissions. Unless you grant unique permissions to the specific item, items will automatically inherit the permissions of the site collection.
=========================== Updated Answer =================================
This push down action is only for permissions, and does not mean that anything I do on the list will also be transferred to the item.
For example, restoring the inheritance operation of the list (Delete unique permissions) does not turn the item from the "unique permissions" state to the "inherited" state
