Why is date modified for a folder earlier than date modified for files inside in Finder?
Folders (on MacOS) are more generally called directories, and that's a better description. Each folder is a list of its contents. A folder is modified when the list changes, that is, when a item (a file or a sub-folder) is added, moved or deleted from the list.

Modifying a folder's files or sub-folders does not change the list, so changing a file does not change the folder's modification date. If a file or sub-folder is added or deleted from the folder, then the list is changed and the folder's modification date changes. That's why a folder's modification date is often older that the modification dates of its contents.

Here are some other questions and answers that mention the same issue.


When does a UNIX directory change its timestamp
Timestamps of a directory?
Does change of a file in a directory not change the time of the directory?