Welcome to the forum, Berni.
I have underlined the subject of the verb 'is' to help make things clearer.
1. 'If there is something I know, it is that....'
2. 'If someone can do this, it is ....'
3. 'w Whatever he's planning, it is in that folder...'
4.'Whatever he's planning is in that folder...'
#3 means roughly Whatever thing he he planning, it (that thing) is in that folder.
#4, which I have added. means roughly: The thing [that he is planning][,whatever that thing may be,] is in that folder.
Student or Learner